Have you done a scan of your computer with your AV program? You should always scan a file before you open it!! What player are you using? Try the files with a different player. Have a try with VLC player, just in case it's a weird codec thing happening
Codecs & VLC

There's a lot of info about virus probs here
WARNING: Viruses on the network you should be aware of
& also a link to Norbiesworld. If you think you have a virus, he'll be able to help you. Go to his site & register (it's free) & follow the instructions for posting a Hijackthis log.