Drop to a command prompt, and run chkdsk on the drive that's missing space. If you're not sure how to do this, click start, run, type "cmd" and hit enter, then once the window opens, type chkdsk c: (or d: or e:, whatever the drive letter is where you're missing space.) Let it do it's thing, it'll take a minute. Once it's completed, look at where it says, "### KB in bad sectors." It should be 0, or very close to it. If it happens to report 111 gigs in bad sectors, congratulations, you've just hit the harddisk size limit in XP. Don't worry though, this limit has been addressed in XP SP1 and beyond. So why are you still seeing this?
Most likely what happened, is when you installed XP, half of your drive wasn't recognized because the original XP has a 137gig (or about that) limit on harddrives, it just don't know how to address any higher. Everything else gets flagged as "bad sectors" on the disk. Even though you updated to XP1, which fixed that little error, the sectors that were marked as bad never get cleared, so XP thinks that half of your harddrive is corrupt. Don't worry though, it's not. One thing you can do, if you can find it, is install from an XP cd that already comes with SP1 or SP2, and format your harddrive to clear the bad sector flags. (Again, nothing is actually wrong with the harddrive, XP just doesn't know what to do with anything over 137 gigs.) If you don't format, those bad sectors won't get cleared.
