VGA + DVI inputs = Diagonal lines?

[sunlitsix]

im using the Hyundai Q17+ monitor, with the PC using DVI input and my Xbox 360 using the VGA.

when ive got both of them running together, i can see diagonal lines on the screen when VGA input is selected. when i unplug the DVI lead the lines stop.
what's causing this? anything i can try to fix it?

 

[sunlitsix]

sorted i think, auto-adjust seems to get rid of the lines

edit : nope the lines just appear again after a few minutes