Slow Computer

I have a user with a computer which works normally, and then about once to twice a day, it slows way down (can take about 20 seconds to bring down a menu), and sometimes locks up. Resources are usually around 65% at the time. She has plenty of hard drive space (around 300-400 MB). Windows is managing swap file. Ran scandisk/defrag. Put Norton Utilities 2000 on it, ran diagnostics – no problems found.

It is an IBM Pentium with 64MB RAM, Novell 5 network.

Any ideas will be greatly appreciated.