Kicked back to lobby without any error

Hey everyone.
I have this annoying problem. I couldn't even play the game yet.

When I try to join a server, sometimes the loading bar stops somewhere in the middle part of the screen. And sometimes the loading completes and I see a peak of the game then all goes black and I get kicked back to the lobby WITHOUT any error messages.

I disabled my anti-virus software (NOD32), I am using administor account on Vista and everything looks to work well until I try to join a serve. No errors are displayed on the screen.

What should I do?