Why won't my computer open any files?

I tried to install WoW on my computer. Everything started out fine, until it got to the most recent update for Vanilla. Now every time I try to open a file, it takes me to and "Open With" window. When I try to open Internet explorer with it, it tries to make me save the file, i try, but then it tells me I can't overwrite the previous file. And when I hit cancel, It closes out of internet explorer entirely. Same with Firefox, except when I hit cancel it actually lets me stay on the browser. I have the K-Lite Codec pack on here, but for every format Windows media tells me that It can't find the correct extension. I don't know what to do or how to fix this and it is really bugging me.