File attribute problem

has anyone come across this problem?
I work for a company which supplies and maintains DOS based EPOS systems.Recently there have been several instances of the hard drive in the tills being modified / corrupted (?) so that using the DIR command produces a file listing which shows the file size of each file as -1 byte and the date stamp as xx/xx/1994.
Is this due to viral activity or is the DOS installation becoming corrupt?.The communications system between the inshop EPOS systems and the company back office was recently changed from ISDN to ADSL using a wide area VPN, could this be part of the problem?

Thanks, Agedcoder