maybe strange XP question

[shorthair]

I was having issues with an old computer.  It was donated to me, and I changed the admin to my name, and deleted the other user account.  When I restarted, it just stuck at some DOS screen.  Nothing I did would help.  I couldn't figure it out.  So I re-installed Windows. Still didn't change anything. Then, one day I was just looking at the case and noticed it had a reset button...and pressed it....and it...reset it! Imagine that.

Well, the other day, I noticed C: said it had 15 gigs used on its 80 gig drive.  I couldn't figure out where.  Then I remember that I put Home over (what I'm pretty sure, as it was an office computer, was) Pro.  I'm not worried about the files I had on it, just the OS itself.  Anyone know an easy way (after I've backed up the files I have on the Home part) to reclaim Pro?