If
this is of what you speak, it's safe enough. But it will prompt you (and maybe bug you) to purchase the full version, and part of it's functioning is likely to be crippled until you do.
Whether it's any good or not, I'm not sure. I've used PCPitstops' online tests before, and after due consideration have decided they are quite good to give you some guidance into what could need doing.
The driver scan, in particular, is fraught. You can download "recommended drivers" and really stuff things up. (If you think you need drivers, it's always best to get them from the support site of the software concerned. If you don't know that you need new drivers, and things are working OK, leave well enough alone.)
Finding the security holes in the browser is worthwhile, for those that don't know. That's a test worth doing, especially if you have been infected before and don't know how it happened. The browser (with it's cargo of flash players and Java)is usually the most likely web-facing application to allow an exploit through, if vulnerable, but you can generally prevent same by keeping it (and Java, and Flash player, patched.)
Other functions in it do a similar but better job than the built-in windows utilities. You can achieve the same performance/state of disk health by using Ccleaner or ATF cleaner, Mydefrag (formerly JKDefrag), and TweakNow registry cleaner. All for nothing. Or just use the Windows tools.
There is nothing the tests do or advise on that you can't find out and remedy yourself using other better - and cheaper - (free) tools. But to offer a reasonably comprehensive all-in-one overview, it's not bad.