Well once upon a time, back in the days of Win 98 SE this kind of problem was fairly commun, this OS simply wasnt able to regognize large drives so it believed that any drive with 2 gigs or over simply had 2 gigs, but Ive never heard of the problem happening with WinXp. Very strange.
Ok this is what I want you to do, reboot you PC and when it first boots up it will automatically detect your drives, I need you to tell me what it detects, ie what type of drive it detects and how big it thinks it is. If your bios detects the drive at 80 gigs then we know the problem isnt that there and chances are you simply messed up the formating somehow (it happens). If on the other hand it doesn;t detect it as 80 gigs then youre going to have to play around with the settings in your bios and possible need to flash it.