Left 4 Dead 2

There is in fact. It says "Disconnected: Session is no longer available. Same error for when others try to join my games. My firewall is not blocking it and neither is my antivirus, the problem seems to be a bit sporadic, coming and going as it sees fit.
And thanks for the welcoming

Indeed, I play TF2, and Supreme Commander LAN and Online with no difficulties.

I shall give that a shot, though the game cache solution was a no go, I'd already tried that. It said I was missing a few files and they'd be restored automatically...I assume they were restored,