Malware

Try disabling Norton and installing the Free version of AVG. I am not particularly fond of either Norton or Mcaffee as they seem to be too all controlling of the machines. I have used AVG on 40 of our systems for almost 2 years now and have never had a virus that shut me down. Wish I could say the same for some of the Spyware programs but thats another story. Just Google AVG. Install the Free version. Actually they will send you 2 emails. The first is a link to the download site. As soon as you begin the download you will get a second email with the activation code you will need to turn on AVG.

If your machine will run XP without Norton try doing a restore back a week or 2 to a date you are sure everything was OK.

While I agree with Rick K that AVG has a good product, the issue could still be a virus. To test this, try to open regedit. If it closes down in the same fashion you've described for Norton, you've definitely got a virus. Some viruses respond this way, shutting down any program that could remove them. This will INCLUDE AVG, unfortunately.

To get around this, reboot into safe mode and run regedit. Look for strange entries in the 'RUN' and 'RUNONCE' areas. Also look for strange file names in the Windows and System32 folders. Of course, this requires knowledge of the Registry and an idea of what Windows files should look like. Google is your friend! Well, that and another computer to use while cleaning the infected one.

Good luck!
Gryf

Let me say this, this does not sound like a virus to me.
I was recently working on a machine for a friend that was showing similar problems. However, they were using McAffee AV, Firewall, etc.
It turned out that there was an udate that they received that went haywire. It took a complete reinstall of the box eventually.
I have also seen similar activities in boxes with Norton.

However, the waiting period you describe between restarts does not point me in the direction of viral activity, unless it has an internal timer that recognizes the BIOS clock. And i have only heard of one virus over the years that had actually obtained access to the BIOS (Chernobyl anyone).
This sounds more like a hardware issue to me. Based soley on the timing issue you reported, and the extra tax on resources that can occur due to running a virus scan. Try, if possible, removing memory and/or replacing it with different sticks and see if this helps.

Sounds like virus to me - check registry for strange entries in HKCM/software/Micro.../windows/currentversion/run
- possible Welchia or sasser virus. I have had Sdbot virus which caused problem. get rid of nortons & install AVG if its virus it will find it