Someone with knowledge and patience please help -_-

I really need some help with my stupid videos. I could play everything just fine until our computer went psycho and my brother reformatted the C drive. Now when I go to Formats in Windows Media Player it says registry error with no message, and I can't get it to play AVIs. I could play those files with DivX until today, and now each one says I need a different codec...thing. It also says unknown exception caught with my .wmv files. I have no idea what's going on and I just want to watch my videos. Could someone please help me out? I'm not a computer buff so I have no idea what to do, is there anyone out there with the patience to walk someone through a Codec for Dummies session?

I know I'm supposed to search for my answer on here and I have, but I just don't understand which solution to use or even what people are talking about. I'm not a techie, I just want to watch my movies. -_-'