Problem solved
For months, I couldn't connect to my SQL Server 2005 database at my ISP using SQL Server Management Studio Express. I thought the issue was firewall related and put off troubleshooting it until last weekend. Turns out the culprit was my AVG antivirus software. I have no idea how or why it was preventing the connection but uninstalling AVG solved my problem. Hope this post helps others avoid the frustration I've endured. Now I need to find an affordable antivirus solution ASAP.