Because you you have to orginal game, just register it to steam and download it there.
Go on google:
search for this,
Alter Iw Net
>Press the underlink, download
And you will have to download the torrent.
(PS: This is just for Multiplayer, not singel player.)
Else, Go on ThePirateBay and search for: mw2.
And find a link by SKIDROW (Or what ever it was again..)
Here is what i searched on google:
and I opend the link here:
Call of Duty Modern Warfare 2 PROPER-SKIDROW (No there is no link, but its the "name of the site" 3th link from top.)