no Microsoft put it there to secretly install viruses /sarcasm/ It is safe but usually doesn't help unless it some level or something. Just do it
its perfectly safe. It won't have any noticeable effect on performance though, unless your disk is REALLY badly fragmented. Its more of a preventative measure than a cure.
There's no real harm in running it. It just won't have any real effect unless your HDD is REALLY fragmented as loony said.
Have lots of downloaded files? Before my OS went out on my large HDD, I had like 80GBs of fragmented data, but it would have taken forever to fix the darned thing. I may defrag it via USB if that would even help it at all.
