I have seen a lot of questions and discussions about monitoring registry values. You can do this with a script. I have written a script that reads a binary value out of the registry and check if the value is the same as the script says it should be, if not it will generate a local event, that Ops Mgr or MOM can pickup and generate an alert on. You can download the script here. You can download the script here. If you instead want to store the value in a textfile, you can see a lot of examples of that here.
For the registry reading part there are more examples here, for example how to read other registry object types.