Capturing bitmaps

I'm writing an application which tells another application to draw a map onto a window. I basically tell the second app (through DDE) to draw to a specific window. I need to capture the output once the drawing is complete for use with OnPaint (to speed up refreshing). I have tried using code found in the bitmaps section of codeguru to capture a window background but unfortunately they capture everything, including dialog boxes which are on top. Ideally I would use a hidden window to draw to and capture off that, which would have the additional benefit of not having to hide foreground windows etc., but I have tried and failed.

Has anyone got any bright ideas?
Many thanks in advance.