I was getting this error when trying to access my SSP. Then I found this terrific forum thread that made my day. Thanks Lionel!
Add the application pool account to the local admin group.
Perform an IISRESET on the SharePoint server.
Run the PSconfig wizard. Test if the issue goes away. If it does, go to Next Step.
Remove the user back from the local admin group, perform an IISRESET again.