I would also look for spyware and keyboard loggers as well as the EVENT VIEWER for clues. You would also be at SP4.
Think you got all the Spyware? Try http://www.pestscan.com/Scan.asp
I have a Windows 2000 server being
used as a desktop workstation.
After about 32 to 48 hours of use the
start button fails to function.
It has sp4 installed and
IE6 is installed with the necessary updated.
Virus scanning and spyware scanning show
The task manager will still activate when hitting
shift-ctrl-esc and the mouse and alt-tab still function.
The machine can be recovered by stopping explorer.exe
and re-starting it.
I would like to find an answer to this problem.