Cannot open file - Windows Vista

I get an error on some computers and not others, all PC"s running Windows Vista Ultimate. The error is Cannot Open File: MK@: FilenameStore:\\ <Folder path> This is a .CHM file I have compiled.

Any idea's if this is a compatibility issue with Vista? The previous compliled file works fine on all systems, this new error just happened after adding more content and re-compliling a new version.

Thanks,