1. Right click MW2 in your Steam library > Properties > Update tab > Untick Cloud Sync.
2. Navigate to: C:\Program Files (x86)\Steam\SteamApps\common\call of duty modern warfare 2\players (if you are on a 32bit OS, there is no '(x86)' in the Program Files folder)
3. Delete every single .stat file you see.
4. Then navigate to: C:\Program Files (x86)\Steam\userdata
There may be one or more folders here that consist of random numbers.
I recommend using the search feature in Windows explorer. You are looking for a folder called 10190.
5. After you have found the '10190' folder, enter it and go into the 'remote' folder.
6. Delete everything inside the 'remote' folder.
7. Open MW2 Multiplayer to see your new rank.
8. Play and rank up to Level 5 and then close the game.
9. Re-enable Steam Cloud Sync as shown in Step 1.