index.dat is a hidden file that contain all of the Web sites that you have ever visited. Every URL, and every Web page is listed there. Not only that but all of the email that has been sent or received through Outlook or Outlook Express is also being logged. Clearing the temporary internet files does not clear the content or the index.dat itself. You need a 3rd party program to clean or clear the content of index.dat
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For years my Cookies folder has held a file called index.dat that can not be deleted nor opened when I clean up my computor. But since it did not seem to be causing a problem I never gave it a thought.
Today out of curiosity I opened it in Notepad wich revealed "Client UrlCache MMF Ver 5.2

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