Mplayer error in NT 4.0

I used to be able to play MPEGs just fine. About a month ago I suddenly started getting errors when I tried to play MPEGs. Generally the error was an access error. I tried uninstalling and reinstalling Windows Media Player. That didn’t help. I have since even installed Real Player and get an error when Real Player attempts to play the mpegs. I have done virus scans, etc. All the typical troubleshooting things. I have searched Microsoft’s web site to no avail. Anyone out there have any suggestions as to why I suddenly cannot play mpegs?

Thanks in advance.