Assuming you have a legal copy of MW2 installed:
Before you rush off to download a random .exe file from a shady link, understand this: Searching for "iw4sp.exe download" directly is a minefield.
Scam sites love this file name. They know PC gamers are desperate to play the old campaign. If you download the first link on Google, you are likely to get: call of duty modern warfare 2 iw4sp.exe download
Do not download standalone .exe files from MediaFire, UploadHaven, or "DLL-fix" websites.
Before you click any "download" button, you must understand what this file is. Assuming you have a legal copy of MW2
In a legitimate, legal copy of Modern Warfare 2 purchased via Steam or the original retail disc, the file responsible for launching the campaign is simply iw4sp.exe. It lives inside the game’s root folder (e.g., C:\Program Files (x86)\Steam\steamapps\common\Call of Duty Modern Warfare 2\normal).
So why are people desperately searching for a standalone download? The answer lies in a catastrophic update from 2018. Do not download standalone
In the vanilla MW2 folder, the single-player executable is simply iw4sp.exe (Single Player). However, the version floating around in the modding community refers to a specific cracked, modified, or client-based executable designed to bypass two major issues:
The modified iw4sp.exe you see discussed on forums strips out the multiplayer bloat and DRM, allowing the campaign to run smoothly at high resolutions and unlocked framerates.