I know you don't want this kind of comment, but have you checked the Windows Firewall is disabled? The only thing I can really see causing the problem in the upgrade. I'm taking it that since you mentioned port forwarding that you've checked only the router side.
I know in W7 the firewall stays a lot quieter than in previous editions of Windows.
If you can get the IP of the server you are trying to connect to, then try pinging that IP from the command line. If that works, but it's still not allowing connections from Steam, it seems to further back the Windows Firewall as a problem, as it blocks connections per program, rather than by port, like your router does.
Hope any of the above can help