To remove existing mods and get back to the vanilla setup:
Delete all the files (but not the folders) at the root of the game's installation, that is, where the .exe is. Inside the packfiles folder, and any of its subfolders, delete any files that DON'T end in ".vpp_pc". Run a cache verification in Steam; it will re-download the files it needs.
Uninstalling it may not have removed files that you added manually, so I suggest giving this a try. If you follow this procedure and it still crashes, I have no other ideas; something else is wrong with your setup.