It sounds like you are trying to boot into an already installed Vista after changing to AHCI mode, is this the case? You might be able to install the SATA/RAID drivers for your chipset from Windows, then shut down Vista and change the mode in the BIOS, and then try booting into Vista again. I don't know if this will work.
Also, I'm assuming that madmatt knows you have an Intel-chipset based motherboard? Otherwise you'd have to get the driver from the appropriate manufacturer, of course. As madmatt said, it's normally the case to already have the BIOS setting how you want it before installing Windows, and load the drivers from removeable media during setup.