After all I started the game.
First time: Game freezed when I switched my weapons for the first time as a marine.
Then Steam was unable to start the game anymore.
Therefore I reinstalled all .net Framework versions on my XP system after removing them.
Then I was able to start the game again.
But every time I join a team in any server but mine the game crashes or freezes without any error message.
It crashes/freezes when the 100% appears.
What's the problem?
I don't understand why it does crash there but not when I host.