OMG PLEASE HELP URGENTLY!!

Hello, yesterday in the evening I went to look at some of my AVI. files on my computer that I wanted to watch but when I click on any of them there is either NO sound or just a white screen!!!!

I don't know what has happened, if anyone could help I would be so grateful!!!!

The only thing I remember doing differently yesterday is downloading an application called 'Speed Download' but I installed it today because it didn't work for some reason.

P.S. MP3. files seem to work as do MP4. and MOV.