XP has gone unstable

I'm trying to diagnose problems with my daughter's laptop that is
running XP Home. Recently the system has become quite unstable and
last night it appeared to spontaneously switch to the 'classic' view
for the task bar and no longer recognizes the modem.

Previous to this I could not get Norton Anti-Virus to run and msconfig
will run for only a short period before it crashes.

I'm going to try uninstalling the most recent updates that I installed
to see if that helps the situation. But beyond that I'm at a loss on
how to attack this problem (other than trying to reload an image backup
that I did quite a while back).

Any help, suggestions, pointers, advice, condolences... would be greatly
appreciated.

Thanks,
Steve