side by side configuration - what does this really mean?

I've recently given an update to a single user who finds that the program now gives an error

"The application has failed to start because its side-by-side configuration is incorrect. Please see.... "

The program runs fine on my machine and older builds runs on his.

However, if I take the older build and re-build, the resulting exe gives the same error.

It's an MFC project and there are no new DLL's. The VC redist is installed on his machine.

If anyone has any ideas they'd be very much appreciated