Had a similar issue on a Sony laptop I own. From the factory, Sony installed some kind of "backup" partition on the drive which saved the setup files for the OS and all of the applications. The partition was not visible anywhere in Windows XP, nor could I see the partitioned files, I just knew my 80 gig drive was missing 15 gigs of storage space. Their theory was, in the event the PC crashed n' burned I could use the "backup partition" to bring the laptop back to factory specs.

Long story short I had to FDISK the drive using an extra Sony utility to reclaim the drive space, then I wiped out the "backup partition" and gained all 15 gigs

IBM might have the same thing on your laptop, check out the original setup discs that came with your laptop (if you still have them) to see if they provide any clues.