i have 5 gigs missing on my drive,, unless you have a clean install of xp you'll find your OLD os still on the drive, that is taking the space up, you could probably delete it, but i dont know for sure what that will do.
Get Partition magic to recover that space... it's not being used. Did you restore from an old image file or something? That could be how you lost the space... not certain tho. You shouldn't have a hidden partition... hidden files and folders yeah, but not partitions.