Converting between coordinate planes

How would you go about converting from a Cartesian coordinate plane to one where (0,0) is in the top left hand corner?

 

Like this:

 

Ok.. maybe I should have put my question a bit better, with a bit more info on what I need this for.

What I have is an object, inside a screen, that is inside another screen. The object can move, and the screen inside the screen is partially-fixed onto the object. If I move the mouse to the left, the screen will follow it to the left, up until the object is on the very right side of the screen which stops it from moving further.

The objects coordinates are based on the large screens origin, and my mouse coordinates are based on the smaller screens plane, which starts in the top left 0,0.

What I am trying to do is to get the mouses coordinates relative to the larger screen.

 

I'm getting there. I started off by thinking that if the mouse was at say, (0,0) then it would be a whole screen distance up and to the left so I would say distance x = Screen size - 0 and distance y = Screen size - 0 up until half of the screen. I realise now that this would only work at the maximums (I think) because as you go towards the middle you would be getting half the screen size, which isn't what I want.

I am about to go to bed, so I will give this a bit of thought while going to sleep and then continue on tommorow.

Oh, one other thing I forgot to mention is that up is -ve and down is +ve, right is +ve and left is -ve for the objects coordinates relative to the large screen.

After a little bit of brainstorming, maybe the distance from the middle of the screen. I'm done. Bed.