Get window handle from mouse, outside form

[Genbox]

I would like to develop a feature from Linux Xserver that makes it possible to resize the currently active window by holding down a key sequence and drag the mouse. My problem is that I need to use the unmanaged Windows API to do that, and I can't get it to work. I've tried to use a global mouse hook and get the window handles from under the mouse, but I could only get X,Y coordinates, and not a usable window handle. Do you have any ideas on how to do this? Edit: I used this to get the X,Y coordinates, but it's only a local hook (but I get a usable window handle, but since it's local, I only get the handle on the form, and not any other window) How to set a Windows hook in Visual C# .NET