Hi Guys,
I believe I may have narrowed it down to the patch KB2596927. The details are here:
http://www.microsoft.com/en-us/downl....aspx?id=45206
I removed the patch in Control Panel, Programs, Uninstall, View Installed Updates on the left.
I've only tried it on one affected PC so far but it's working in Excel now with the ActiveX controls.