WPF popwindows open in the upper corner of the screen

We're creating a WPF application with VS2010 and .Net 4.0. For some reason on one computer (XP sp3) all pop windows display in the upper left corner of the screen. The pop windows I'm talking abo开发者_开发百科ut are tooltips, menu's, combo box lists etc.

What would cause this?

Thanks,

Brad

I think we came to the conclusion that it's a video driver problem. The application works on all other PC's.