EXCEPTION_ACCESS_VIOLATION and Connection Broken

We have received error messages in Query Analyzer doing table alters with no one else on the system. The message stated that the connection had been broken. In Event Viewer, the following message pops up:

Error: 0, Severity: 19, State: 0
SqlDumpExceptionHandler: Process 23 generated fatal exception c0000005 EXCEPTION_ACCESS_VIOLATION. SQL Server is terminating this process.

We did a dbcc checkdb, and there are corrupt table messages and a message like the one above logged. We have restored from a backup of taken early this morning after having no errors with dbcc checkdb, checkalloc, and checkcatalog. Then, we did a dbcc checkdb after setting the dboptions to single-user mode (using sp_dboption) and received table corruption errors and the same message logged in Event Viewer. There was a connection broken message when we tried to get out of single-user mode.

I have seen a postings about this, but I really do not know what to look for. We are on version 7 SP1. What else should I check?