Straight from Valves Website:
If your account has been banned for past cheating violations, VAC (Valve Anti-Cheat) has detected that your account used a cheat while connecting to a secure (VAC-enabled) server. Cheats negatively effect everyone's playing experience and we have a zero tolerance policy with regards to use of cheats.
Valve's Anti-Cheat system (VAC) automatically detects programs and other methods used to cheat in Valve's games and does not have any false positives in the system.
We will not un-ban you regardless of the reason. It doesn't matter if someone else used your account, you didn't know what you were doing was wrong, your brother or sister downloaded a cheat you didn't know about, etc.
Use of your Steam account is your responsibility.
VAC may not ban you immediately, the cheating violation may have occurred weeks earlier.
Length of Ban: First time offenders are banned for one (1) year from playing any of Valve's games on secure servers.
What can I do if I get banned?
You can still play single-player games or on non-secure servers through Steam (you can filter the server list in Steam to show "Insecure Servers" only (from the main Steam menu -> Servers -> Anti-Cheat -> "Insecure Only").
If you are going to purchase a new copy of the game, make sure you delete all previous versions of Valve's software prior to doing the install and make sure you get rid of any cheats you've installed (unfortunately, this might even require re-formatting your hard-drive).
If the cheat still exists on your system, your new install will get you banned again.