CoD4 problem, help needed

Hey folks,

Since this morning I get some strange errors when connecting to servers and when the server is changing maps. When connecting, I get the windows error sound when having my screen on "Waiting for other players.." and then it refuses to do anything. When changing maps, I get the error when it's loading the map.
A reboot is needed in both cases to make my computer work again. Though, when I rebooted, it works fine, but when the map changes again, it gives the error. Anyone has got a clue why my computer is doing this?

Thanks in advance.