Network access by .NET applications

Why would all my WinForms applications (running with FullTrust permission set) be unable to access remote resources? All non-CLR applications seem to be working fine.

I'm not behind a proxy (I am using NAT though), but the message is something I've seen before when dealing with misconfigured proxies at work.

The underlying connection was closed: Unable to connect to the remote server.

I'm going to spend more time later trying to figure this out, but I thought someone else may have run into this before. Any ideas?

Engineering