Don't worry about it.
Recently I upgraded my PC and also went from XP to Windows 7 Ultimate. I copied all my contents of my HDD to another HDD and after reinstalling, copied them back. Of course, I had to reinstall programs but miraculously, MW2 was the only game that I didn't have to reinstall. Somehow it instantly ran on my new OS. All data (saved games, multiplayer) was there.
Recently I upgraded my PC and also went from XP to Windows 7 Ultimate. I copied all my contents of my HDD to another HDD and after reinstalling, copied them back. Of course, I had to reinstall programs but miraculously, MW2 was the only game that I didn't have to reinstall. Somehow it instantly ran on my new OS. All data (saved games, multiplayer) was there.