hooking into WM_PAINT event of an application

Hi can anyone show me how to hook into the paint event of an application for example calculator.

What i would like to do i add additional functionality when the screen is refreshed.

Thanks Matt.

What I would do is:
1. Subclass the application's main window.
2. Look for WM_PAINT
- Pass message to original window function, then add your own drawing afterwards.
3. Pass on all other messages wholesale.