I used the Registry method to change from IDE to AHCI yesterday on my Vista 64-bit system. It worked fine for me. In fact Microsoft has the instructions and more information here:
Error message when you start a Windows Vista-based computer after you change the SATA mode of the boot drive: "STOP 0x0000007B INACCESSABLE_BOOT_DEVICE"
You need to carry out this Regedit procedure, reboot, go into your BIOS, switch from IDE to AHCI and things should work fine. All the usual disclaimers apply!