Fixed position/size windows in an MDI application

You should process WM_WINDOWPOSCHANGING message. Basically you write a single handler for this message to process each window of your MDI application. LParam parameter of the above message points to a WINDOWPOS structure, one member of which is called flags (UINT flags).

if you write something like this in your message handler:
WINDOWPOS *ws = (WINDOWPOS*)msg.lParam; //msg is the message
ws->flags &= SWP_NOMOVE; //retains the current position
ws->flags &= SWP_NOSIZE; // retains the current size