Quick and simple, if after loading the game crashes to desktop in offline training, simple do this:
1 – Goto steamapps\common\alien swarm
2 – Delete the folder called “bin”.
3 – Go Back to steam, click “Steam” > “Settings”.
4 – Downloads + Cloud Tab.
5 – Change your download region to ITALY.

Restart Steam, right click Alien Swarm, Properties, Verify Cache.
Let it check, after some seconds it will popup a message saying “70 files are broken” or something like this. Let it download (~30MB). If you’re a girl, go make me a sandwich in the meantime.

After finished, enjoy your free game. Maybe some files are corrupt on other download servers.

This worked for me like sandviches making heavy ohnomnomnom.
(Core i7, 24GB’s DDR3 1066, GTX480, Win7 x64 and steam installed on a network drive).

Pics or didn’t happened.