Use this guide if you are having trouble getting the Modern Warfare II achievements to unlock in the new game.

1. Install Call of Duty (HQ)
2. Right click Call of Duty (HQ) > properties
3. Go to DLC tab and untick all options (this will save download space)
4. Once downloaded, install the new Modern Warfare II game (if not already installed)
5. Boot up Call of Duty (HQ), navigate to the Games tab and launch Modern Warfare II

The achievements will now start to unlock. Enjoy.