Odd Crash Bug

[SecondRaven]

Here is the problem:

If i join a game that someone is hosting i will get through all the loading and get to the ingame screen. Yet it will crash like 2 seconds after i get "in game" so i am not sure whats going on there.

You might say that its a video bug right? Well thats what i thought but now this:

If i host my own game i can get into "in game" but no one else can. I have turned off my firewall and unluged my router. What else is there to do?

Thankyou for your time and help