This is very much after the event. I no longer have the problem - see later on, but would like some feedback as to just what might have happened.
One day, everything is working just fine (XP Home, SP3, fully patched, ZoneAlarm firewall, Norton AV).
Next day, system starts up, all Startup programs work, but as soon as I try and do anything CPU jumps to 100%, mouse freezes, clock stops, and I can't do anything at all. Reboot, exactly the same behaviour.
Boot into Safe mode, everything (that can) works OK. Try again in normal mode, freezes again.
Back to Safe mode, use Explorer to look in Program Files, Windows, Windows/system32 for any files made up of random letters/numbers - nothing.
Run full Norton scan - nothing. Have an AVG download, install and run that - nothing. Download Avast, run that - nothing. Run AdAware scan - nothing other than a few tracking cookies. Run SpySweeper scan - nothing. Run SpyWare Doctor scan - nothing.
As it's in Safe mode and working, not much point in running HijackThis.
Try a System Restore to a few days before, restore works but no change, still freezing. Back to Recovery Console, do "fixmbr". This gives the message "This computer appears to have a non-standard or invalid MBR".
Still no joy.
Find installation disk, do a "Repair" on existing installation. Windows comes up and starts working - for all of two minutes, then freezes again (and again and again).
Finally, after checking that I have everything important saved on external USB drives, cut my losses, full reinstall after wiping c:\ drive. Everything back to normal.
After all this, can someone suggest what might have happened? (Hopefully it won't ever happen again, but it never hurts to be prepared).
Obviously, the old system no longer exists so I can't run any further tests on it, but if there is something that can be run in Safe mode that might pick up on any future occurrence I'd like to know about it?