If you are looking for the "complete feature" version of the game using the IW4x mod client plays a specific role: Downgrading
If you do not have the base game installed, you can download a pre-patched version: call of duty modern warfare 2 iw4spexe link download
Assuming you have downloaded the safe iw4sp.exe link, here is how to set it up correctly. If you are looking for the "complete feature"