What neurolysis says USED to be the case. In practice, I cut this on all the time in Windows 7 - it's painfully simple and non-complex compared to Windows XP to go from IDE to AHCI.
If you want to change 7 from IDE to AHCI, all you need to do before you change the BIOS setting is look in the registry at HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\services\msahci and change the 'Start' key to 0. (If an AHCI driver has never run on this machine it's probably 3.) The first time the machine boots in an AHCI controller mode it should load the default AHCI driver properly and prompt for a reboot.
Note, however, that depending on your drivers and controller the REVERSE is not always true - it may work just fine if you change from AHCI to IDE, or you may get bluescreens. (I've never had occassion to fix one of those so I couldn't tell you how.)
EDIT: I would just add that I have no experience with this in Vista but I suspect it works much like 7. In XP, changing this is a huge undertaking that involves a major registry hack tailored to your specific hardware, and a reinstall is almost always simpler unless you're going to be doing a dozen identical machines or so.
DOUBLEEDIT: There is a very minute chance of failure here in the event that you have a really strange motherboard that won't work properly with the default MSAHCI driver and will ONLY use its own. Try installing it first if that's the case. (I have done this on dozens of current-gen boards and NEVER seen one that wouldn't run with the baked in driver, and in fact it benchmarks very well.)
