To get rid of the "Steam cannot contact key server" error:
1. go to run and type in regedit. click ok
2. go to HKEY_CURRENT_USER
3. go to Software>Valve>Steam>Users
4. there should then be some folders with random numbers
click on each one and go to friends and under PersonaName it should have your account name. do this to find the account you are using to play whatever game
5. click on cdkeys
6. go to your specified appid and if yours isn't there right click and go to New>String Value and enter in the appid*
7. double click on the appid and enter a random number for the value data
*to find your game's appid check here:
Steam appid's (use ctrl+f and type in your game to find it) if there is more than one for your game use the first one or the one that is a more even number like 15600 instead of 15602
that will make the error go away but you will still probably need to download a cracked launcher to play the game. try finding one here:
Cracks
Edit: i'm pretty sure l4d2's appid is 550
this crack looks like it would work:
Left 4 Dead crack
(sorry its a torrent)