Also known as simply Windows Search, Windows Desktop Search is based on the old LookOut search tool. Microsoft bought LookOut a couple years ago, and their first try at incorporating it into their MSN Toolbar product was horrible. They finally got it right!
Windows Desktop Search will, like LookOut before it, index the contents of one's hard drive, one's Outlook (both PST's and Exchange inbox), and network drives (now via UNC paths). However, it will now allow dragging and dropping, from search results windows. It will also allow multiple search results windows and the ability to sort them on various fields. One can choose a dynamic "as you type" hit list, or one can de-select that (as I did).
Installing it, and uninstalling LookOut, will greatly increase Outlook's response and load times, because the tool is no longer an Outlook add-in.
To download the tool, go to http://support.microsoft.com/kb/926356.
For information on trouble-shooting and configuration, see http://www.microsoft.com/technet/prodtechnol/windows/search/dtstshoot.mspx#ESC .