If you have blue screen when starting Windows 7 after you enable AHCI, try follow the below step to disable AHCI and force windows to reload/reinstall AHCI drivers on next boot.
- Disable AHCI in BIOS; turn HDD configuration to IDE
- Click Start, type regedit in the Search box, and then press ENTER.
- Locate the following registry key
- In the right pane, right-click Start in the Name column, and then click Modify.
- In the Value data box, type 0, and then click OK.
- Reboot your computer and boot into BIOS setup
- Turn on AHCI in your HDD configuration