Windows and some applications create a large number of rubbish files, it waste your hard disk space. These rubbish files let your computer slower and slower! When you visit internet, Windows record all internet activities which you had visited and all information which you had browsed. Someone may be interested in these information. When you delete a secret file, the file doesn't really disappear from the hard disk and intact exist in hard disk. If there are some scheming persons, they could find these files by some hacker's programs.