1. Registry Alert
Registry Alert is the best free easy & Reliable Registry monitoring tool, it allows users to protect their computer's sensible database (reg) from different malicious programs. Once installed, it will be running reduced and in stealth mode. Whenever a new program (malicious or not) demands access to registry, the program will prompt you with a Yes or No buttons to allow or not the program to access and modify your registry. If it's a trustworthy program and it was ran by you then simply click Allow to enable registry access.
2. Monitor Registry Changes Software
Monitor Registry Changes software is a little different from the above monitor and listener tool Registry Alert. This software is no doubt a registry monitor but works in a different way, it takes a snapshot (current image of the registry) and starts listening for any new entries and - in realtime - comparing the current registry to the old created snapshot, if a difference is found then it will lists it. Another feature of this software is that you can monitor (listening) on a specific branch (example: HKLM only or HKLM/Software/Microsoft ) what will make it easy to find any suspicious access.