Service Pack 1 crashes

For the past several weeks, my system has been crashing
and restarting every several hours (irregularly. Crashes
seem unrelated to anything application-wise -- it does
the same thing whether I am working on it or not.

Seems like it should be a virus or worm, but three
different scanners -- including a boot-level scan -- have
found it clean.

The only event log "crash message" is

Microsoft (R) Windows (R) 5.01. 2600 Service Pack 1
Uniprocessor Free.

For more information, see Help and Support Center at
http://go.microsoft.com/fwlink/events.asp.

which is the last event before the crash...needless to
say there is no info on this problem at the help link.

Restarts are usually normally, occasionally XP wants to
check a couple of the hard drives but they always come
back error free.

Restoring to dates before the crashes started has not
helped, restoring "way back" might, of course, help at
the price of knocking out dozens of programs and drivers
that were working fine at one time.

This thing is driving me nuts. Any ideas? Thanks.

Oh, yes, we have an Athlon XP 2200, 1.5 GB DDR RAM, ASUS
A7N8x2.0 Delux MB, 3/09/03 Phoenix BIOS Release, ATi Rage
Pro Video, two ATA ultra 133 drives, one SATA drive, a
Plextor DVD .....
..