I've been battling the dreaded too.
Fairly old amd Athlon 2000+ with XP Pro which keeps spontaneously restarting or locking completely requiring reset.
On restart, windows goes through the usual 'recovering from a major error' routine.
I've tried restoring to an earlier time, registry cleaning, complete scan with AVG, and then Avast, updating drivers.
Then the reinstalling of windows and still the same problem exists.
I guess it must be a hardware issue, needing the old faithful to be replaced.
Any ideas before I fork out wads of cash?