quite simple though,
go to:
C:\Program Files\Steam\userdata
and then your account, you can mostly recognise it if you check which folder has last been changed. then in your accounts folder delete the .bak files.
go to:
"C:\Program Files\Steam\SteamApps\common\call of duty modern warfare 2\players
delete all .stat files
i'm not sure if it works for alteriw too but for normal cod it defenitly works, but have a try