One thing you may have forgot is that Windows computers keeps files called index.dat. Unless you are a advanced user, like the advanced Nokia hacker, you cannot easily delete these files. These files are used by Windows Explorer as a database which runs as long as Windows is active. This file stores all of your search queries, web sites you visited, autocomplete, Internet Explorer history, cache, cookies, and recently opened files. These files remain even after a restart, and also after you clear any of your histories! These index.dat files can be use to recover what you opened, what you looked up, and what you typed into forms even if you think you have it erased.
Why are these files there?
Well, some people might say convenience and speed. But why does windows lock these files, and why is it so hard to delete them? Why are they hidden from the system? Everything is passed off as a feature to better help you live, but its true nature may be to invade your privacy.