Call of Duty: Modern Warfare 2 Discussion
2. Go to the Start button and select Run (XP) or type Run in the Search box and choose Run from the list (Vista).
3. In the Run window type:
"C:\Program Files\Steam\Steam.exe" -install E:
Replace E: with the CD/DVD drive you are installing from if is not correct.
Replace C:\Program Files\Steam if your Steam installation is not in the default location.
4. Press OK. Your installation should continue from the disc.
I just found this after a quick search on the steam forum, try that...
* Updated game trainers and cheats daily
* Get notified when new cheats are added
* Request which games get new trainers
* Priority support with any problem