Possibiltties and fixes include:
1. It has unpartitioned space, or non-windows space (like
LINUX). If so, use the XP disk management tool to either
create a new partition (if free space) or remove partition
and then recreate/reformat if non-windows.
2. It has one partition of 80 Gig, but yur motherboard's
BIOS is too old to use more than 32Gig, and the 38Gig you
mentioned is merely one type of flakines that can occur.
If so, look for a BIOS update to fix this problem. If no
BIOS update, think about an ATA/100 PCI adapter card and
hook the disk to that.
3. Drive is really 40 Gig, which rounds out to about 38
Gig depending on whether the definition of your Gig is
octal or decimal based.
4. In any event, consider using a more fundamental tool
than XP to examine the disk to see what is really there
(assuming no BIOS limitation). I like SeaTools by
Seagate, a free download. But, there are other similar
tools for free on the web.