Enable the DMZ and check all your firewall stuff.
Before we go jumping the gun on this one (I've heard about port trojans and other viruses coming in from enabling DMZ), shouldn't I post my anti-virus software, maybe it might be causing problems.
I have the firewall up, but most of the programs I run servers on are allowed through.
I also use Microsoft Security Essentials. I dunno, it's never given me problems before.
Also another thing that is weird. I can host minecraft and tekkit with ease, but nothing else...