. While it's the executable that launches the single-player campaign, downloading it from third-party "DLL fixer" or "EXE download" sites is highly discouraged as these files are frequently bundled with malware or identified as Trojans.
The only safe and legal way to get iw4sp.exe is to install the official Call of Duty: Modern Warfare 2 (2009) game via a legitimate storefront.
The AlterWare Singleplayer Guide provides instructions on using a "downgraded" version of iw4sp.exe to allow the AlterWare Launcher to run the modded single-player client.
Steam will scan your folder, realize iw4sp.exe is missing, and download a clean, official copy automatically. "iw4sp.exe is Missing" - Common Fixes
Steam will scan your directory and automatically redownload the original iw4sp.exe if it is missing or corrupted. Select the game in your library. Click the Gear icon (Options) next to the Play button.