Notebook screen blacks out

Hey everyone:

I've got a old Sony Vaio notebook that seems to blackout or hibernate
erratically.

Specs:
Win 98 (not SE)
PII (did I say it was old?)
64MB SDRAM

What happens is that once Windows boots, it can be anywhere from 5 minutes
to 2 hours later and the screen will go black and freeze although the fans
are still running. It doesn't ever do this in Safe Mode, and I've disabled
and removed a bunch of programs (Anti Virus, etc.) Is this an OS thing or a
potential memory problem? It's my mom's computer, so I might have to search
for spyware.
Any thoughts from the board would be great.
Adam