My steam account has an underscore ( _ ) in it, but you can't use underscores in your username for the forums.

 

When I run iw4m.exe, I get this:

 

[NP] Initializing RPC

[NP] Sending RPC message with ID 1.

[NP] Sending async RPC message with ID 1.

[NP] Sending RPC message with ID 0.

Performing Steam authentication...

 

 

Error during initialization:

Your current Steam account does not own Call of Duty: Modern Warfare 2. Please purchase the game or try another Steam account.

 

I do own the original game, and I have 182 hours on it.

Is it because my forum username is different to my Steam account username?

Set your steam profile to public, and then try again.

 

Your username has nothing to do with it, it scans you steam game's list.

Thanks! It worked

This answer should really be someplace obvious, like in a FAQ post, stickied to the top of the forum where people can look before posting....

 

Oh wait. It is. viewtopic.php?f=7&t=4