I inject the .dll file into the iw5m.exe so I think it is in my Steam folder.
If I delete the local data from MW3 an install it again, is the hack deleted too? (Sry for my bad english xD)
As a previous post has suggested, it would really depend when steam had scanned your computer. If it's been scanned, and you decide to delete the local data and reinstall, the steam account attached to the Game ID would have been scanned, therefore causing what is known as a VAC Ban. Files are scanned through online play, mostly multiplayer and limited singleplayer modes, if anything is detected, you will notice a constant kick from online servers the next time you play.