There is a DIBAPI32.DLL on the microsoft site which contains a few api calls which can capture :
1: The whole desktop
2: A window (given a handle to it)
3: A client area of any given window (Without the borders and title bar)

Then it can save the capture to a BMP file.

Search the on support site from M$ trough the knowledgebase for the following keywords:
WINCAP, DIBAPI, DIBAPI32

One of the articles contains a sample with the DIBAPI32.DLL Another file contains the sourcecode (C++) of the api (Search for WINCAP). I've worked with this DLL for a few days now and it works fine.

In combination with the Wang Image Edit Control (OCX) you can edit this bitmap and save it to other formats (16, 256, 24 b color, JPG, TIFF, etc)

Succes