if you were banned the only thing you can do is appeal to valve - it most likely won't do any good however.
http://steampowered.com/forums/showthread.php?s=&threadid=88879
you must have had either yourself use hacks, or someone log on to your account from your computer (a friend maybe) and download hacks - there is no other way - if your account was banned by accident then valve would unban it asap.
Plus, VAC just updated so it is most likely it detected something fishy on your computer and banned you.
"hack detections are logged and banned later - again this is deliberate deterrant to cheats, as rather simply you may think you are getting away with hacking when in actual fact you are not.
Valves policy is not to respond to trouble shooter queries if there is a positive detection
PS - false positive are very rare and in cases in the past where they have occured Valve have been VERY quick to reverse those errors."