Are you using a drive that already has a windows install on it?
I had a similar problem on Wednesday with my brother's new system, it turned out the Vista (full version) installer was detecting a previous install of Windows on one of the secondary hard drives (I'd put his old drive in, as well as his new drive).
Disconnected the drive that he'd previously used as his boot drive in his old machine, and windows happily installed to the new one.