Laptop Freezes

I am working on this for a friend. Gateway 4538GZ Pentium M 1600 mhz.

After updating WinXP, updating drivers to clear event log messages,
cleaning out Norton AV (uhg!!), it boots without event errors, and
runs some, but freezes after 15 min to 6 hrs or more.

I ran HD mfg test program, no errors.

I am running MemTest86, at 15 hours, no errors.

Any thoughts?

Phil