Right click on the steam icon in your active tray
- Go to games then highlight Call of Duty: MOdern Warefare 2 Multiplayer
- On the bottom right hand, click on Properties
- In the Properties go to the "Local files" tab
- At the very bottom click on Defragment cache files
- One that is done it will give you a "Defragment is complete." message, click on close
- Now click on option "Verify integrity of game cache..."
- It will take up to 10-15 minutes and if the files are not okay it will automatically download the right files and place them in the right place (you will get a confirmation message)
Note: just incase, back-up your profile before doing this.