Modern Warfare 2 : Multiplayer - Cant' Launch

Hello,
Thanks for your time, this is quite a long story.
I bought Call of Duty : Modern Warfare 2 on steam. Multiplayer would start up but would not load any maps. Single-player wouldn't even launch properly (the title screen from the game box in a small screen would pop up but the game would not start).
We tried to fix the problem but to no avail.
Then I had the idea to buy the game on a disc. I bought the disk and it installed (However it automatically installed onto Steam) to the computer and I loaded it up.
Multiplayer would now load up and start matches and I got to level 7. I went to sleep and in the morning it wouldn't even load up Multiplayer (the title screen from the case in a small screen would pop up but the game would not start).
Now I have tried many things and it doesn't work.
Once again thanks for your time and I hope you can help.
Bye!