[Bloody Sam Roberts]

WARNING! I think this is a HOAX! DO NOT delete index.dat until you speak to your system manager!

Good advice but it's not a hoax in that the information is correct about the file growing in size. It shouldn't just be deleted though. In fact it CAN'T be deleted when IE is running.
There is a utility you can download called delindex.bat. It takes some configuration to do it right and is definitely not something to be tried by anyone not familiar with the process of setting up .bat files and Windows startup.

I've used it on a number of Win 98 type machines and it does make IE load and run faster. Nothing more.

[Steven W.]

Here's another case where Microsoft bigots and a variety of stupid, paranoid or idiotic people without a single sense of technology get all in a huff about something totally benign they have no understanding of. Simply put, browsers cache pages and portions thereof to improve performance by being able to retrieve a local copy of a file and other content rather than having to download repeatedly. In order for the browser to ascertain whether the file or other resource has been previously downloaded and to locate where such was previously stored a persistent "index" file is used. If you don't want to take advantage of the caching feature then disable it through the browser. If you want to clean up temporary internet files that are no longer being utilized then it's preferable have the browser do it for you to ensure that both the index and associated cache files are deleted in tandem. Next thing you know these same "geniuses" who are trying to undermine the browsers caching capabilities will be complaining about its download times or other problems they've created.