I cant tell you if this solves the problem but you can try it.
If you are getting different resolution on .NET's
System.Windows.SystemParameters.PrimaryScreenWidth
and
System.Windows.SystemParameters.PrimaryScreenHeight
than your current resolution settings, you might wanna check out your viewing settings.
To do so, go to your desktop, right click, and select "Screen resolution".
Then, click on "Make text and other items larger or smaller"
After this, click on the Smaller - 100% radio button.
it solves the problem for me once but again , cant say if it will solves your problem. hope it helps