This bug stems from older versions of L4D2 not playing nice with newer processors. Because this mod is abandoned it has not received the same updates as L4D2.

What you want to do is press the windows key + r and search msconfig.

















Open it, go to boot, go into advanced options and decrease the number of processor at 19 or less
















Restart your computer and attempt to launch the game again

If you're still having issues try putting -windowed in the launch options other than that, that's all I have for you.
I sourced all information from this guide from the Valve Devloper Wiki whilst trying to find this solution for a video I was working on. I had no luck finding this information in any relevant forums so I wanted to post this here. Hopefully this will work for you as it did for me.