I have experienced a similiar problem on many sites when they are running Norton Anti-Christ, er Anti-virus. The solution has been to 'exclude' files of file type .dbc, .dbf, .cdx, .dcx, .fpt, and .dct. This has to be done on all workstations.

One note - it seems that periodically, this excluded file list is lost (maybe with Norton updates) and the procedure has to be performed again. these days, I suggest ANY other anti-virus software to my clients, as I have not seen this problem with any others.