I encountered a very bad bug (forgive my French, awesome French)

I had a localhost server in 1.6.2 and wanted to test it back while i was using 1.6.4 for another online server... I thought that minecraft was at last multi-version but it's not. I installed forge 1.6.2 over 1.6.4 then everything broke. Now, whatever if i uninstall minecraft or even try alternative launchers, mods that worked fine doesn't load anymore. Optifine, dynamic light are just a few to mention. Also, i don't know why but most of the mods people succeed to install fails to load on my computer while it's much a beefy-computer...

 

I think about reinstalling windows because i can't figure what broke and it's the only way to ensure a fresh system to work with

 

Any clues why everything that broke won't fix after i reinstall?

1.6 is no longer support, update.