If you can connect this way, it's possible the shortcut link in your Start Menu is corrupted.
If you cannot, it is possible that the Wupdmgr.exe file is missing or corrupted.
The solution to both of these issues can be found on Microsoft KB Article 193691.
If this does not resolve your issue, there is a registry hack that may help. Check out MS KB Article 228548.
I also seem to remember a worm that did this a while back. I would start by updating and running your antivirus software and doing a system scan.
This was first published in March 2005