Movie doesn't load on some PC's

We have a flash movie on our homepage:

It loads fine on Macs, but is inconsistent on PC’s.
On some PC’s, even with the latest shockwave player installed, all that displays is a Shockwave icon.

Is this some kind of problem with our Publish settings or the EMBED/OBJECT properties in Dreamweaver.
We are not currently using a browser-detect script.

Any help would be appreciated. Thank you for your generosity-