In a thread on vac on steam forums, I see that you can join servers vac enable without a lot of players by clicking on view servers and check secure servers while there is somes mw3 servers. I wait bo2 like this to buy it. See this thread here http://steamcommunity.com/discussions/forum/9/882959527688934488/
It's called valve anti-cheat information thread, the best one I see since the start of mw3.
To install or reapair steam, go exclusively here http://store.steampowered.com/about/