Virus Scan

I have a customer with an IBM PC.  Pentium 4 and 128MB running Windows 2000.  I tuned it up over the weekend and went to run the System Virus Scan (Norton Antivirus 2002 with subscription good until August 2004).  The last date of a full system scan is Oct 23, 2003.  I tried to run the scan manually and it got 895 files into the scan (still in the I386 folder and slowed to a crawl.  For the first 895 files i saw the numbers go up a hundred or so at a time.  After 895 the number advanced one file at a time.  I left it running overnight (over 12 hours) and the scan was still running.  It had scanned 45k files and saw no viri, but was still scanning.  Still only one fiel at a time as well.  And of course the system was very slow to respond.  I stopped the scan and ran a file error check and did a defrag.  I still got the same results.  I uninstalled Norton and before reinstalling it I ran a program from McCaffee called Stinger to scan for viri.  The system is clean. Stinger ran the total system (145K files and folders) in about 20 minutes.  Then I installed Norton Antivirus 2003 and tried another scan.  12 hours later I had to stop the scan and return the computer to the customer so he could get back to work.  I'll pick it up this weekend and try again but I'm not sure what to try. I’m thinking of installing the 2004 version and if that doesn’t work, then doing a dirty install.  Any ideas?

Thanks

Thedslguy