Can't Connect to LAN games, and ideas?

I’m just at a loss here, so I need some ideas. my friends are over and we’re trying to play a game of Stronghold over LAN.

So far:
I can see their games.
When I try to connect to these games it locks up (like it’s about to connect) and then bumps me back to the connection screen.
They cannot see my games.
They can connect to each other.
I can do anything else LAN related, i.e. ping them, this is the only thing screwing up.
All relevant ports are forwarded, all firewalls deactivated (and exceptions added when not deactivated)
UPnP is on.
NAT is open.
No DMZs or such to speak of.
My Laptop can connect, my desktop cannot. They are virtually identical, down to the operating system.

This is driving me nuts, I’ve tried just about every workaround and tweak I can think of, and need some ideas.

Are you behind a router?
Are the ports for Stronghold open for your router, if it has a firewall?
Is your connection ADSL or cable?

Cable, and yes all necessary ports on the router are open.

Edit:
However, since this is LAN, I’m not certain how my external internet connection comes into play.