Listen, mate. I have been banned from MW2 for over a week now, and neither I used any hacks.
Tho' there are peeps that made a hack so that people can create dedicated servers, because this new COD doesn't originally have them. So, if you get randomly directed into a server like that (even if you didn't want to), VAC detects this as a hack after a week or so.
This is what happened to me, and so I realised that it sucks. Modern Warfare 2 for PC is shit, and money waste. I am happy now playing it on PS3, and got my nuke yesterday. Don't worry, just get it on another console or buy a new one. Steam and Valve do not talk about bans. They just tell you to buy a new one.
I feel sorry for yo.