The pages on MSDN should explain all. In a nutshell:
You open the key to monitor
You create an event
You set the watch for the registry key for a change of value
You wait for the event to occur
You close the key
You close the event
But If you don't understand those MSDN pages, then I'm afraid that explaining this word by word what you must do is way and far too much. What you ask is certainly not for beginners and you need a very big baggage of VB knowledge (and good API knowledge).
If you still want to know how, I suggest to start reading every page I linked to (and subpages!). And only then do a google search for code which uses that API function.
Google
RegNotifyChangeKeyValue vb6... eg:
result
(but note that this case is certainly not a case where you'll simply be able to copy/paste some code without understanding it bit by bit and without reading every link posted in code and threads; also because many code you'll find is buggy and/or very specific. And using this API has also many hidden caveats which you'll only be able to understand if you have a very good knowledge about all the used supporting APIs).
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