This comes straight from steam, btw I am banned in MW2 too and I still play MW3.
The Valve Anti-Cheat system is enabled for servers displaying the Secured shield logo. Using custom skins, sounds or maps and playing multi-player mods which do not modify core .EXE and .DLL files will not result in a VAC ban
Cheating in a Source Engine game that was released prior to Left 4 Dead will result in a VAC ban from Counter-Strike: Source, Half-Life 2: Deathmatch, Day of Defeat: Source, and Team Fortress 2.
Cheating in a Gold Source engine game will result in a VAC ban from Counter-Strike, Condition Zero, Ricochet, Day of Defeat, Team Fortress Classic, Half-Life: Deathmatch, and Deathmatch Classic
Call of Duty: Modern Warfare 2
VAC is enabled on all multiplayer servers, public and private. Any modifications to the game other than editing config files can result in a VAC ban. Examples of such modifications include but are not limited to console enablers, VAC disablers, and server hosting mods.
A VAC ban in Modern Warfare 2 only affects Modern Warfare 2.