There have been issues with setting the SATA mode to AHCI, where XP wont recognize the drive. So if that happens to you, you will need to use it in IDE mode. Although from what i've heard, if you can you should run it in AHCI mode for a bit better performance.You have to switch the controller to AHCI mode. I don't think it will boot into windows unless you install windows with it in ACHI mode.
If you are running XP, you will need to reinstall Windows if you switch to ACHI from IDE, or vice versa. Although Vista has the appropriate controllers for both so you most likely can switch in and out without needing to reinstall.You enable AHCI in the BIOS but as Omega said, it doesn't always agree with windows. If you do change it from IDE to ACHI I'm pretty sure you will have to reinstall windows otherwise it will just blue screen when you try and boot. By all means give it a go without reinstalling if you want to switch to AHCI but it might not work.