Well as the subject says, I'm unable to host co-op mode whilst in Windows 7. Running this under my XP partition works fine.
Info that may help
-Multiplayer works (Dedicated server)
-Firewall (ESET Smart Security 4) set to interactive, have allowed friends IP's.
-Using Hamachi 1.0.5.1 (newer versions don't work with this)
OS: Windows 7 Ultimate 32bit
CoD Version: 1.0