XP home, fully updated.

What can cause a computer to shut down without warning?

I have run memstest86 but the RAM is all OK. Also run a total virus scan
using McAfee (which has always been installed and running) but no virus was
found. No new hardware has been added recently. And I have not noticed
that I was doing anything that could be a common cause of the shutdowns -
they seem to be random and sometimes allow me to work all day and other
times it shuts down after 5 minutes.

I have noticed that I regularly get a disabled desktop waring saying there
was an error 'object doesn't support this action' in line 65 of desktop.htt
but it is the same desktop I have always used and it loads fine in
FrontPage.

Any ideas what to look for next?