Exporer.exe does not started by default

Hi,
I have a system with win 2000 professinal installed. About 2 days before I started my system and when I logged on a blank screen with mouse pointer appeared. I opend the Task Manager and in the new task I ran c:\winnt\explorer.exe. Then my desktop came before me.
Now the problem is that whenever I log on I have to run explorer.exe . Could any body tell me How can I get rid of it and my system would come to its previous state and explorer.exe run everytime bydefault.
Thanks in Advance.