First, let me just say that sometimes it could be the server that is different from the client, but if this keeps happening there are a few things you could try:
- In Steam, select the game, then
Right Click->Properties, go to the
Local Files tab, and click
Verify Integrity of Game Cache. If steam finds any outdated/missing files, it will inform you and start downloading them.
- Browse to the
steamapps folder, locate your copy of MW2, and inside
/main, delete (or cut out) all the
iw_XX.iwd files. re-run the verify integrity. It should pick up the missing files and re-download them.
Finally, the most drastic:
Right Click->Properties, go to
Local Files, and
Delete Local Game Content. Once it is done, go into
steamapps and make sure there are no remaining files. If there are (shouldn't be), delete them. Then re-download the game.
I got stuck with the last option on MW3, it sucked, but luckily I had the game discs (I bought it retail). I just installed off them and then let steam run updates.