PC locks up

Has anyone experienced their PC locking up during idle time? I am running Windows 2000 Pro with Avast 4.6 installed. My PC occasionally locks up after several minutes of idle time. I am not sure if it is related to Avast but the problem seems to have started shortly (perhaps a week) after installing the software. I’ve scanned for everything Avast can scan for, I’ve run Norton Utilities, Ad-Aware, and Spybot to correct any possible problems. I would appreciate any ideas as to what I can look for. Thanks.

Besides Norton Utilities, are you using Norton Antivirus? Did you use in the past? Are you sure you proper uninstall it? Cleaned the Registry?