This Is a simple guide to create a "Quick and Easy" dual boot system. It will use the integrated boot selector from Darwin... You will need:
1. (Surely) Mac OS X 10.4.6 JAS Install DVD (on a DVD-R not RW!)
2. (Surely) A PC with a compatible DVD drive (to install OS X, it's necessary!)
3. (Maybe!) Windows XP Install CD
4. (To avoid criticism of this guide) be sure OSX can run on your computer before attempting anything like a dual boot!
It's simple and quite easy to understand but you must either:
a. Start from scratch with an empty HDD (longest but easiest and requires item 3)
b. Partition your disk with a program (as I don't use these, I will not cover them here; there is sufficient info on that elsewhere)
c. Start from an already partitioned Win XP system with sufficient space to install OS X on an unused partition. This "Quick and Easy" Solution took 25 minutes for me to get OS X installed and running with a boot selector (installation of OS X included!).
To start, you should have two system partitions with at least 10 GB each to get comfortable with Virtual Memory (both OS), the hibernation sleep mode of Windows and the bunch of program you will install to use your PC/Mac to work/play/whatever. Important: A partition must start below the 1024 cylinder on the disk to be bootable. Make sure the partition designated for OS X starts below the 1024 boundary.
We will start from scratch; others (case b and c) will get onboard later.
1. First of all, put your Windows XP Install CD in the drive.
-When the installer will ask your where to install Windows, select your HDD and create one partition of at least 10 GB for Windows XP and keep space (but do not allocate it for now) for an OS X partition of at least the same size. Use FAT 32 for Win XP System because OS X can read/write on these drives but not on NTFS (read only). Leave the rest of the disk unused. (To be honest you could even set up the entire HDD from Win XP install but it's easier from OS X).
-Install Windows on the partition you just created, Windows will always make the install partition the active partition and then boot from there.
2. Setup Windows (The longer part!)
3. (we take case b. and c. people here!)
Put the OS X Install DVD in the drive (Yes, really!)
-When you are at the installation menu, choose from the menu Utilities -> Disc Utility.
-Then choose your disk and then select your unused Space and setup your drive from here and select the Journaled MacOS format for your drives. (I will not explain the use of the "Disc Utility" so look for guides about it but I say that this utility is VERY user friendly)
-When you setup, make sure that your Mac OS X drive is set to PRIMARY because if it's not the case, the computer will not boot on that partition! Also, put a label in the label section without special characters such as é à ù or "space" because it will be easier to call from the command line.
-Then, be sure to have your partition mounted (it's said somewhere, but I don't really remember) as you will need to access it from the terminal.
-Quit the Disk Utility, this will get you directly to the install menu.
The next step is needed when you use the darwin bootloader (gave me the problem that i didn't have time to select my OS (chain0 method works fine)
-Then select, from the menu, Utilities -> Terminal. You will get to the tiny command line of Darwin (as someone said: don't panic, the terminal is your friend!)
-Then type: "diskutil list" (as always, without quotes). Keep note of the EXACT DISK/PARTITION where your MacOSX partition is, it will be something like: disk0s2 (Disk 0, partition #2 here).
-Then type "fdisk -e /dev/rdisk0". The 0 is the number following the "disk" in disk0s2. Hint for the Noobs: replace the number, here 0, with YOUR OWN MacOSX disk number!
Note: you will be in the sub command prompt of the fdisk program.
-Then type "p", it will list the partitions of this HDD. Verify the number of your MacOSX partition just be sure that it will be your OS X system partition and not another you will make active. The active partition (here Win XP) will be identified with an "*".
-Type: "flag 2" where the 2 is the partition # in the "disk0s2". Hint for the Noobs: replace the number, here 2, with YOUR OWN MacOSX partition number!
This will mark your partition as the active partition for the boot sequence at PC start-up.
-Type: "write" to write your changes (it will ask you if it's OK to set the changes on the next reboot, indeed, say Yes).
-Type: "exit" to exit fdisk program.
-Type: "exit" to exit Terminal.
-Close terminal with Command (ALT)-Q.
-Install OS X with the right options for your PC (go the install FAQ to get some info on that). Reboot and upgrade with the update pack to 10.4.8 or 10.4.7.
-If you have a dual core cpu then you have to disable one core. You can do this in the Bios. (updated by joost :)
-Reboot and enjoy the boot menu of Darwin when you press F8 while booting. You can select any OS and voilà!
Note 1: If your boot is too fast and prevents you from choosing your boot partition, then you should edit your com.apple.boot.plist file to wait a little bit before booting.
To do so, go in terminal and:
-Type "sudo -s"
-Type "cd /Library/Preferences/SystemConfiguration/"
-Type "pico com.apple.Boot.plist"
add (or replace the number if the strings exist)
-Type "exit" to exit sudo
-Type exit to close the terminal
reboot to check
This will make your OS wait 8 sec. before booting. You can change the time to your convenience. Other tricks of the boot option can be seen here.
Note 2: The Boot loader of Darwin is accessible by pressing F8 before the white start screen, by default, there is an 8 sec. delay. It looks like a command line, but you can select your starting partition with the up and down arrow keys. It's a real boot loader for the volume you are on; it can select any partition to start. Also, there is NO need to change your active partition once set on the OSX partition, the boot loader will transfer execution to the partition you select but will retain its ability to be the active partition on the HDD (the first one to be read on the boot sequence).
In case it goes wrong and the partition doesn't boot, just access the terminal from the DVD and repeat the Terminal procedure with the right partition. But to be sure it's not because your partition is not a primary one, delete it with the Disk Utility of OS X to the "unused space" state and create it again, just in case.
Also, if OS X doesn't boot at all on your PC, then your PC would be unstartable. Just put the DVD in the machine and repeat the Terminal procedure to change the starting partition to Win XP, but this should be an unlikely event as you are able to boot from the DVD.
Notes 3: It could certainly work for more OS like Linux, OS2, DOS or any obscure OS but I haven't tried, so I could not write it here, but as the Boot selector seems quite flexible, it should work.
 Alternative Suggestion
If your disk's boot sector is corrupted by this process (for example, the Disk Utility forces you to "partition" your drive, erasing Windows XP, or a subsequent install that claims NTLDR is missing), you can try what I did to get this working.
1. Begin the install for Windows XP. When you get to the disk partition screen, delete ALL partitions on the disk. After the disk is completely raw and unpartitioned, power off the computer and reboot into the XP installation.
2. At the next partition screen, create two partitions. In my case, I had a 57Gb (60 billion bytes) drive; so I created a first partition of 27gb and a second of 30gb. I chose to install Windows XP on the first. Choose your first partition, and select quick format FAT - it will warn you that it's using FAT32. This is okay. Finish the Windows XP install.
3. Install your JaS 10.4.8 disc. When the install comes up, select Utilities->Terminal. Do "diskutil list" and find the 2nd partition that you created during the Windows XP setup. Mine was disk0s5, but it could also be disk0s2 or something else. It probably will claim to be a 16 Bit MS DOS or Windows partition. No matter.
4. We need to re-create the second partition. First, label the disk with "disklabel -create disk0s#" where # is the number identified above for the 2nd partition. Then, type "diskutil eraseVolume "Journaled HFS+" OSX disk0s#" -- note, you WILL include the quotes between diskutil and disk0s# because the "Journaled HFS+" is a single parameter. Note that # in disk0s# is the number that you identified in step 3. Issue a "diskutil list" again and notice that your partition is now an Apple_HFS volume with the name OSX. Hit CTRL-D to logout of Terminal and ALT-Q to close Terminal.
5. Proceed with the OS X install - the setup will now recognize the OSX volume and allow you to install to it. Select the relevant options and continue on. I suggest following PANIC's tip for extending the wait time for alternative boots.
This method worked for me on a new IBM/Lenovo T60 model 1951-4TU.