Error message when you start a Windows 7 or Windows Vista-based computer after you change the SATA mode of the boot drive: "STOP 0x0000007B INACCESSABLE_BOOT_DEVICE"
(How to switch to the AHCI driver
using an existing Windows installation.)
You say nothing about your hardware. Windows 7 X64 includes AHCI drivers
for some chipsets (like my Intel X58), but I don't know whether they support all systems.
As far as I've read, AHCI offers pretty limited benefits in real use, so you won't be missing much if you can't switch.
If you have access to a PC with a DVD drive, the Windows 7 DVD, and a USB flash drive (4GB), I suggest preparing a bootable USB key drive: Use a USB Key to Install Windows 7?Even on a Netbook
(What sort of system running Windows 7 X64 has no DVD drive? Unusual.)