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.
EP, glad to see you come back and tidy up...did want to ask a one day favor, I want to enhance my resume , was hoping you could make me administrator for a day, if so, take me right off since I won't be here to do anything, and don't know the slightest about the board, but it would be nice putting "served administrator osnn", if can do, THANKS