If your missing 23 Gig! go to your admiinistrative tools, storage and check to see if only 137Gig where set active, and the 23 Gig left inactive. Activate it, format it and make a new drive. You mention you have SP2. Did you download and install it after the initial installation of XP (which edition)? because XP along without the SP2 would only recognize 137 Max.
Now with SP2, it can recognize hard disk up to something like 4 Tera!