Though DAT files, which house data as the shortened name implies, are usually best left alone, here's how you can open them.

Windows only: Index.dat Suite is an index file editor that allows you to selectively edit or completely erase the contents of index.dat files on your computer.

I would like to empty the contents of the INDEX.DAT file in C:WINDOWS/Temporary Internet Files/Content.IE5. This file has grown to 496 KB in just four months. I find this very disturbing.