I doubt a virus, as William_Wilson said, CPU usage being high when the machine isn't doing anything is generally a sure sign of that. Is your CPU fan working? I had a rig that was acting like yours and it was because the thing was overheating and just locking up. If your fan is good then do a virus scan (if you can) and see what that throws up, but I don't think it will be that.

The other thing I can think of is programs running in the background. Have you installed anything recently that runs in the background of Windows (it should be in the syatem tray next to the clock) sometimes these programs run and are not needed, close any that are running but not in immediate use and see if that helps.

If you can install Mother Board monitor and FreeRAM XP, FreeRAM will show you your RAM usage and is very acurate.

Let us know how you get on!