As the title says, I have the steam version and it doesn't launch the game, stating it is unavailable.

I am running on vista with dx9 and dx10 support.

AVG detects the Crysis exe as a threat also.

Go to .../steam/steamapps/common/crysis2/installers and reinstall everything in that folder.

You may also need to disable AVG as it may be quarantining the Crysis 2 executable due to a false positive.

I Just tried what you said, and no luck

I had to uninstall my AVG. I could not get the game to start until AVG was completely gone.

Alright, I will try that and report back