Thanks for the help so far!
Now, one QA person is showing odd visual problems that prevent her from using the Windows Themes; she has to remove the manifest to make the application work.
I've been consulting the MSDN and I have been checking versions of Shell32.dll, UxTheme.dll, and Comctl32.dll with no conclusive results, yet.
An example of the webpage I've consulted: http://msdn.microsoft.com/en-us/library/ms649781%28VS.85%29.aspx
Attached is a screenshot of what she is seeing (our login window). (That's her desktop showing in the text field and button areas). Any ideas?