Perhaps by accident. Here's some frequently asked questions and answers on how to deal with the situation. Most browsers such as Netscape Navigator and Internet Explorer save images from visited Web sites on your hard drive whether you click on them or not so that next time you visit you won't have to re-download the image. But this means that many megabytes of images of all types get saved to you hard drive. Put it this way, if you saw the image then it's on your hard drive.
Browsers also store visited web site addresses in the pulldown address bar.
If switched on, the AutoComplete function will type out previously visited sites if you type the first few letters of the address. To learn how to turn AutoComplete off, see further down the page for the question that addresses that problem specifically. You can do it manually or you can use a software solution. Check out Evidence Eliminator software , a very smart and thorough product that will save you from doing it all manually. It cleans not only your browser, but registry and swap disk, something that cannot be done manually with any ease.