Mouse position relative to the screen? i.e. how far away the cursor is from the top of the monitor? I'm not too sure how this would be useful though... or, you mean, where the mouse exists in the 3d space? imho that'd be hard to tell too, cuz a cursor only has a 2d co-cordinate (no Z), and u generally can't really, control or define where the cursor itself is, in a 3d space.
Why not just position it from another window?