Registry is one of the most important aspect of Windows and if you are doing any change on Registry, make sure that you do a back up of it. Registry Live Watch is a free, lightweight and portable tool to monitor activity on a registry key. You can run this tool from the system tray (notification area in Windows 7) and monitor a registry key for different kind of changes. The application in itself does not change the registry, but just monitors in read only mode.
Registry Live Watch is a portable application, there is no install process, just unzip to any folder and run it. You need to specify the root key from the available drop down list and enter the key value. Once you click monitor, it starts the monitor of the particular key.
You can also save the log of monitoring. You can also specify to run an event in case there is any change in the keys you have selected for monitoring.