A guy I know is having trouble playing L4D2 on Steam because it is saying he needs to remove his mods and plug-ins before he is allowed to play Online. He owns Oblivion on Steam and says he heavily modded Oblivion. He also states that his Valve/Steam directory is not located in the Program Files folder which leads me to believe his Valve folder is in the same directory as his Python folder.

My question is this: Could Steam pick up Python and/or Oblivion and have that interfere with other Steam games? I know the VAC system runs the mod/plug-in checks all the way back to the directory in which the Valve folder is located and if the Valve folder is located outside of the Program Files folder, it can pick up foreign files and claim them to be mods/plug-ins. This is usually because nobody ever installs mods directly in their Program Files folder so it rarely ever happens. My old roommate had to move his Valve/Steam directory into his Program Files(x86) folder because Steam found some mods he installed directly to his My Documents folder and prevented him from playing games.

If you guys need more information, it might take a bit as I'm communicating with him via a message board. But I'll give updates as soon as I get them.

Does he have mods installed for L4D2? If he doesn't, he needs to contact Steam Tech Support. One game should have zero affect on another game.

In a word...no. I have a very, very heavily modded Oblivion set-up with Wrye Bash, OBSE, and the whole shooting match. Steam hasn't even batted an eyelash over it, much less prevented me from playing other games. (And since I have 800 hours logged on my copy of Oblivion, you'd think it would have objected by now.)

Also, I have Steam installed to a non-Program Files location (C:\Games\Steam, in my case), and VAC has never pitched a fit over any foreign files anywhere. Basically what I'm saying is: VAC should really not be getting upset over anything in his \Oblivion folder. If he has mods or custom maps for L4D2 (or any other Source game) installed, have him remove those and see if that fixes the problem. It's possible one of them has a cheat or exploit enabled.

If he doesn't have any mods/maps for L4D2, or any other Source game, then I'd do what HeyYou suggests: talk to Steam support.

By the way, if he's able to play online after removing mods/plug-ins, I'm not sure it's the VAC system that's getting upset with him. Last I knew if you went into online multiplayer with a cheat or exploit running (on a VAC secured server) it was banhammer time; no warnings given.