1) Start Steam and go to your games library.
2) Right click on the game you want to verify and click on Properties:
3) Go to the Local Files tab and click on Verify Integrity of game cache...
4) After the process is complete, you will have a clean, vanilla copy of Modern Warfare 3. You will need to re-apply the 1.4.382 and TeknoMW3 patches; simply follow the normal installation instructions but skip step 1.