If data not backed up, now might be a good time to buy a new (larger) hard drive. Replace existing drive with the new drive. Install XP, (if your disk doesn't include SP2, would recommend borrowing a CD from a mate that does; not illegal cos you'll install with your serial number).
Install software, esp. antivirus (in case that was the problem on the old drive).
Shut down, set the jumper on your old drive to 'Slave', plug it into the connector half way down the data cable and find a spare power lead for it from the PSU.
Start up and you should have a Drive D as well as Drive C.
Run a virus check on all drives.
You can now copy any documents from D to C. When happy you've got everything, reformat Drive D.
I'd then recommend setting up windows so that your 'My Documents' folder is on Drive D, not Drive C. That way you're protected from similar problems in future - if Windows screws up, or slows down, or gets generally clogged up with crap, you can reformat your C drive and reinstall XP and other software without having to worry about your documents.