INSTRUCTIONS IF YOU HAVE THE GAME INSTALLED FROM PREVIOUS ACCOUNT:


1. Create new account or add to existing account that doesnt have MW2

2. PLAY!!

If the above doesn't work try this

1. cut the MW2 folder located in \Steam\steamapps\common\call of duty
modern warfare 2
   and paste it somewhere like desktop.

2. Log into your new account and activate the using the cdkey, pause
single and multiplayer.

3. paste the mw2 folder back into \Steam\steamapps\common\call of duty
modern warfare 2

4. restart steam

5. launch the game and play