To draw to GDI within a thread

Hello,

i have a thread running with a loop of methods, they are drawing to
the GDI (not GDI+).

But there are no results on screen. It is like i would draw into a
null-device.

I've tested the following soultion: dont draw itself within a thread,
but changing in this thread some parameters and call InvalidateRect.
On WM_PAINT my main-program draws with the changed Parameters.

But if there are more threads than one, it is very strange what
happends.

In my draw- and change-routine i've testet EnterCriticalSection and
all that, it doesnt matter.

When i have a simple thread with a untimed loop of rectangles-Calls,
there is nothing happen!

What do i make wrong?

Kind Regards,
CyanSky