I work in software support and this problem is probably a issue with their Vista image, because it's not really "our" software as client can't access any xml files at all

Client reports that on any Vista OS in his office when he tries to access XML files through our programs xml viewer, it "just flashes" and nothing (his XP pcs work just fine).

Navigating through explorer to the xml produces same results. I tried to use the event viewer and client couldn't view xml friendly results either. We have never had a call like this and I have asked several developers and googled my fingers off and can't find a solution.

I just hate telling clients to reimage so if anyone has any ideas they are appreciated

Identify an alternate XML viewer and have them install/try it. If they can view XML docs on that PC with that program, then the problem may actually BE your program's XML viewing code.