Using the Win32 API you should be able to use the EnumWindows() and EnumWindowsProc() functions to retrieve the handles to all windows on the screen. If you're searching for a specific window, you can use the FindWindow() and FindWindowEx() functions and pass them the title of the window you're looking for. After getting the handles, you can use functions such as GetWindowInfo() to get titles of the windows or other info.

Check MSDN for all of the helpful functions that are along these lines.

-Jeff