Detecting windows update events?

Question

Hello. I am working on a sort of "PC monitor/assistant" application program (.NET on Windows XP) which detects visible changes in the appearance of windows associated with desktop applications. I have tried taking a snapshot of the desktop when my program starts, then periodically taking snapshots and comparing them with the previous snapshot via pixel-by-pixel comparison. That method will miss important events and is computationally very expensive. Perhaps a better way would be to detect invalidate, refresh, paint, etc. event messages being sent to the other application programs, but after having looked at Microsoft docs and Win32 API calls haven't found a way to do it. If the second method is a good way to get the job done, how do I do that? I conjecture that "virtual desktop" apps do it, or something similar. If that's not how it's done, how is it done?