So I built my first gaming PC, and after going through some problems I was finally able to get to install Vista. Everything was working fine...Windows was starting for the first time...and then my PC reboots and says "Hard drive is not detected!" which doesn't make any sense because I just installed vista on it. Any idea what might be causing this?

hopefully just a loose cable. check both ends of your power and data cables to make sure they're in properly and snug. also, if you have problems after checking cables, try another SATA or IDE cable to connect your HD to the mobo. they just go bad sometimes.

good luck

Check your BIOS to determine if the HD is detected.

If detedted then maybe the drive priority is not set to your boot drive. This is not the same as boot order, it is another setting.