Useless control

Hello everybody!
I'm student in Computer Sciencies and my prof asks from me to write ActiveX control, but ... it's a bit hard for me. Let's explain what I have to do.
The final result must be ActiveX control, containing two picture boxes. One pict. box is static - the other can be dragged. There is random lines inside both picture boxes. You can drag the moveable picture box freely and you must see its contents. When you drag it over the static picture box, the movement must stop just in case if lines inside two picture boxes will cross. Difficult to descript it... For example, if both boxes are square and there are only one vertical line inside each, dividing boxes in two parts, the result must be: when dragging second pict. box, it can overlap the first, but must stop move if user want to drag it so it cross the line(s).
If someone could help me with some algorithm... A, my prof told my the function BITBLT could help...
Than you!