Argh! Help!

I have a file that I have created on a Macintosh. Which works fine on a macintosh - but not on any of the other PCs around the office.

Essentially, the crux of the problem is that there's a load of duplicate movie clips flying about. So as to mask them off into a neat area, I have a movie clip that sits on top of them all...

Now I send that movieclip to the far reaches of the stacking order by this command:

duplicateMovieClip ("_root.mask", "topmask", 9999999999999999999);

This works just fine on my precioussssss Mac, but when the SWF is opened on a PC - it just doesn't work and all those randomly duplicating movie clips appear over the top of my frame.

What gives?