A customer has a 30 User Network of XP Pro PCs, fully updated with Winodws Updates
Yesterday 2 Users had the error "This file is not in a recognizable format" when attempting to open Excel documents but it was NOT all Excel documents, one User had the error on 2 files & one User on just one file, other Excel files these Users accessed were ok - I was unable to repair the files & the users had to restore from an earlier date
Today I've just had a call that another User has one file reporting the problem

Has anybody come across this ?
Does it sound Virus, Trojan etc ?

Any help would be appreciated, thanks