'Let's play Heroes 5 online.'

Hello. My friend and I wanted to play Heroes 5 (version 3.1, T.O.T.E.) through Hamachi, but neither of us can see each other in the lobby. He has Vista, and I have XP. We moved Hamachi to the top of the list, but it still doesn't work... My firewall is disabled, and I have Avast antivirus installed (it's expired).
He has the firewall enabled, but says it's configured and that it's not the problem. His antivirus is Dr.Web.
What should we do?: D We have the same version of Hamachi.