Why would you think it not safe to use?
If it is not the full version it may not be full functioning but will not screw over your system merely show you what is slowing it down so what have you to lose?.
personally i dont use any system tools for windows except mozillas built in cleaning of cookies etc and defragmenter in system tools,unless you`ve been naughty in how you add and remove programs or have installed naughty programs your registry should not need to be fixed and it is quite easy to adjust the page file size so that it doesn't use up more than it needs,that is about all any of these system tools do and you can do it yourself without adding another program to do it for you.