What happens in your code when you click on the browse button. Does it use an external program for that?

I have found that the behavior of windows 7 is that it assesses files before it opens them and may be asking for user intervention (hard to spot sometimes because it just lays down in the taskbar). OTOH this might not be it at all but crank the UAC of windows way down (you can't turn it off anymore)

When I went fromVDF7 to 15.1 I found several errors and behavior changes. Upon investigating each one it always came down to sloppy programming that was somehow forgiven in VDF7. But my bet with your description is that it is all UAC. Lastly if they installed 7 on the old XP machine there will definitely be slower performance