It might very well be that your motherboard is too ancient to recognise bigger HD's. Sometimes (usually?) flashing a newer BIOS ('Kickstart') into it can help. Do you happen to know what motherboard you have? We might be able to help you find a newer version.
It's a bit like the 4 gig limit on Miggies, but not quite :-).
Oh and if that wouldn't help, I'd be happy to swap my 15 gig for your 80gig :-D.