Extremely important note: Window XP is VERY touchy. If the drivers are not installed properly or you do not have the correct driver installed Windows will not boot; it will simply bluescreen at startup. If you are having trouble finding basic items such as the "Device Manager" I STRONGLY advise against performing this mod on Windows XP as you will be up ****'s creek without a paddle if something goes wrong. With that said, the "Device Manager" is in the System control panel.
I am not sure how reliable the Window 7 support in the MBR mod script is as I have yet to test it. It works fine for Vista x64 though.
If you are using XP you first need to boot into Windows. Once you are in you need to replace your existing IDE drivers with the Intel AHCI ones (downloaded from Intel's site, the link is in the InsanelyMac thread). Windows will yell at you that the drivers are not supported on your hardware but you should ignore this message. Once that is done, reboot into OS X and run the attached "ahci.sh" shell script (must be run via sudo). The script will spit out each of the drives that it finds with a Windows MBR, just select the one you want to mod. Once that is complete you should be able to reboot into the Windows install you just modified and have AHCI support.