Does anyone have specific knowledge of a Windows virus that modifies/eliminates the association of image files on Windows?

I am helping in the administration of a Windows network and I am getting many complaints from a certain group of users claiming that they are not able to open .JPG files anymore (they all use to be able to open them without problems). Their Windows system (Win95) does not recognize the format and does not associate it with any programs. If I take the file to another PC on the same network (but in another group of users) it opens without problems.

Since the complaints are coming from one group of users I was thinking that maybe they have been infected, through an email message addressed to the group, by some virus that got away from our antivirus software (NAV).

By the way, I have tried to force file associations on these Windows PCs, and I still can not open these image files.