maybe the video?
when vista came out, it was made a big deal that you had to have a certain amount of video memory or video card, etc...
The GX280 might be a little too old to run vista if you have only onboard graphics. I've run vista on gx520 before, but the gx520 is a generation ahead of the 280. ( i mention vista because windows 7 is obviously "based" on vista and will have similar requirements )
How much RAM do you have?