Tell me what branches of the registry I need to watch and analyze, so that the program has a universal application?
It's not that simple. Some time ago
I posted a proggie that is supposed to play videos.
The good news is that on my Win7-64 it can handle wmv, mkv, webm, swf, flv, mp4, mpg, avi and asf. Plus the audio formats cda, mp3, mid and wav.
The bad news is that on your pc it may handle nothing at all. It depends entirely on the codecs installed. I have worked hard to understand this business, and all I can say is that
it is an incredible mess 
My proggie uses DirectShow. Before I tried the older MCI, and all I can is "they are different". They are both from Microsoft, but codecs that work on MCI will not work with DShow and vice versa. It is really a can of worms.
What you achieve with ShellExecute is simply to launch the player (WMP or third party, e.g. VLC or FLV) that
the user associated with a particular ending. Example:
include \masm32\MasmBasic\MasmBasic.inc
Init
ShEx "sample.swf"
EndOfCode
If the file exists, after a few seconds Firefox opens and shows me a boring blank page with "Activate Adobe Flash" in the middle. If I click on that link, FF asks me again if I want to allow Flash to run, and if I agree (I shouldn't

), after a while I can see the video. Great.
In this respect, DShow is more versatile because it picks automatically a codec if it is installed, i.e. no user intervention needed; and you can construct your own GUI around it.