The database is SQL Server 2008 Express running on a Windows XP Pro SP3 PC. The database can still be accessed from the server PC itself and another Windows XP Pro PC. Attempting to connect from 2 Vista workstations, both Ultimate, but one 32 bit and the other 64 bit, now results in an error:

Connection failed:
SQLState: '01000'
SQL Server Error: 53
[Microsoft][ODBC SQL Server Driver][DBNETLIB]ConnectionOpen (Connect()).
Connection failed:
SQLState: '08001'
SQL Server Error: 17
[Microsoft][ODBC SQL Server Driver][DBNETLIB]
    SQL Server does not exist or access denied.

The same thing happens whether we use a trusted connection or the sa login (just for testing to check if the problem was a permissions issue).

I intended to post the solution to this issue here this week, but I haven’t found one yet. My hope was that if this was a result of a Windows update, I would find more information about it this first week of January as more people returned to work and others ran into the same issue.

Update:

I found the solution to this by simply debugging the problem based on the symptoms. All information I found about this error talked about opening ports to allow SQL Server access. The culprit was the Windows Firewall on the Windows XP Pro PC hosting the SQL Server database. One of my test steps was to temporarily disable the Windows firewall and this made everything work as before.

So, I enabled the firewall again and added exceptions for TCP port 1433 and UDP port 1434. This resulted in a different error (HYT00 “timeout expired”) during he initial login attempt, but worked on subsequent tries.  I was able to resolve this error by allowing outgoing time exceeded messages via ICMP in the Windows firewall settings.

None of these exceptions or allowances were ever enabled before this problem started, so something changed to cause this to happen. My money is still on some Windows or SQL Server security update. I wish I had more time to track this down further and find the culprit, but I’m happy to have a working solution now.