Excel files on other PCs

I use the same version of Excel on two PCs at work with XP and one at home with Vista. I have copied files between PCs with out any problems for quite some time. I have recently started to have problems with files created on one of the work PC's opening them at home. I get the message "Excel found unreadable content in file......." followed by the message that Excel was unable to repair the damage.

Files from the other work work PC open OK and copying files between the work PC's isn't a problem. The work PC is not connected to the internet so it is unlikely to be a virus.

I have tried lowering the macro security setting to the lowest without result. Any ideas as to what the problem is and how to resolve it please.