Windows XP Home Edition

OK, It sounds like those drives have been partitioned, and that your only using the first half of the disk. The second part of the disk is most likely unallocated. First of all, can you go to My Computer, Control Panel, Administrative Tools. Double click on Computer Management, and click on Storage, followed by Disk Management. On the bottom right hand pane, you'll see your drives shown here, and the parition information of each one. Can you see them split into two, with the second part of each one showing as "Unallocated"?

I don't see any unallocated partitions.

The first drive is split into two: HP Recovery 4.41 GB FAT32 EISA, and (F 4.9 GB NTFS
The second, HP_PAVILLION (C is just one partition, at 9.31 GB NTFS.

Are you able to do a screenshot of that page for me. Also can you look in your Device Manager (Control Panel, System, Hardware tab). Under the category is "Disk Drives", what do you see?

Device Manager and Disk Manager screenshots.

OK, since you received that PC, have you ever seen those as 20gb (18gb) drives?