If you have installed your Windows 7 with the AHCI turned off in BIOS, but out of a sudden AHCI is required (for example to run OSX) it is still possible to change the setting without leaving Windows 7 ubootable.
According to http://support.microsoft.com/kb/922976
It is required to change two registry entries BEFORE you make any changes in BIOS
- Click Start and in the "search" field enter regedit, then press Enter
- If User Account Control dialog shows up, allow Registry Editor by clicking Continue.
- Find the following keys in registry:
- Find the "Start" variable in the right pane, right click it and choose Modify.
- Change the value from 3 to 0. Repeat the same steps for both services mentioned in step 3
- Close Registry Editor.
- Reboot your computer and press DEL or F12 to enter BIOS
- Change the controller mode from IDE to AHCI
- Choose "Save and exit setup", confirm with a 'Y' if necessary.
- Done, your Windows 7 should boot without problems.