No desktop in XP pro

After I log on to windows, explorer does not run. I can
go to task manager (16 processes are running) but cannot
run new ones. Just get the msg “This file does not have a
program associated with it for performing this action.”
Same thing in safe mode. I have done hardware
diagnostics, chkdsk and virus check (computer is visible
over the network)and found no problems.

Has anyone encountered this problem.