Can't open Applications

After starting the PC and using it for about 30 minutes the quick launch icons and the START/PROGRAMS options stop working. That is, I can't start another browser window or open Outlook (or other apps) from the quick launch bar
and trying to select an application from START/PROGRAMS doesn't work. It doesn't seem to matter how many windows are open.

No error message or anything. Just no response.
However, I can go to the Office toolbar and continue to open applications from it.

Thesituation only corrects on a reboot and is good for another half hour. Any ideas?

Standalone Win2k Gateway desktop with IE6.0