is it a slot a socket for the cpu? if that is the case it is a very old pc, 1999-98
in that time a 40 gb hard disk was recognized as 8 gb hard disk most times and a 80 gb hard disk was recognized as 20 or 10 gbs
bios on those days were very limited by the cilynder and head concepts from old systems
if you can create the partitions you need on another pc and then connect the hard disk to the athlon it could be enough
the suggestion made by menotu can help too, also gparted livecd can help you
this apps can bypass bios and do the tasks "talking" directly with the hard disk and avoiding the bios limitations
}if the space reported by the system is correctly, a 40 gb hard disk is seen as a 40 gb hard disk then the problems partitioning could be caused by two reasons
1. errors in the mbr or in logical sectors/partitions, this can be fixed by a low level format, the manufacturer tool to wipe the hard disk can help you, samsung, maxtor, hitachi, seagate and others will have this tool in their websites
2. hard disk failures, on the surface of the platters or in the logical controller, in both situations is easier to replace the hard disk, it is practically dead