I see so many people on a daily basis destroy their systems with this and similar tools, especially concerning the registry cleaning and registry defragmenting.
Always back up the registry first before doing any sort of cleaning in there and it's recommended to have a system image and repair disc on standby just in case the xxxx hits the fan, if people don't understand how the registry works then stay well away.
Another thing to mention is cleaning the registry has yet to be proven to give any benefit whatsoever and a much safer method I'd suggest is to use a good defragmenter once every couple of weeks and run a tool called " CCleaner " at the end of every days browsing before shutting down the system ( defrag and CCleaner should be all you'll ever need )
Note that CCleaner also has a registry cleaning option but it doesn't go very deep into the registry so it hasn't caused issues in my experience but it also gives the option to back up the registry first before running or simply don't use that feature if the registry is an unknown evil to you.