CCleaner is a marvelous freeware program that deletes the numerous unnecessary disk files cluttering your system disk. The typical CCleaner run will free up a half a gigabyte of disk space. It's safe, I run it every few weeks and never a hiccup.
In this day and age of 200 Gigabyte hard drives, why do we care? Zapping clutter off the hard drive makes file searches go faster, makes anti virus runs go faster, makes Windows Explorer load faster and paint new screen faster. It makes browsers go faster. Viruses (Virii) malware has to hide on your hard drive somewhere. It's easier to hid in a few gigabytes of clutter.
Google will find you a download site for CCleaner. The site will attempt to sell you a fancier version that costs money, but the free one is all I need.