Yes, I do mean  WINDOWS EXPLORER - NOT - INTERNET EXPLORER (IE).

You can install this to add tabs to Window's Explorer on Win XP or VISTA        .
Tabs can be defined so that you can reach down to your sub-folders with one click,
which is why I love it.  Tabs can be created in groups, and groups can optionally
open on start-up of Windows Explorer.

The UI is still rough compared to Internet Browing in IE7 and Firefox.
Also each tab seems to require about 10MB of memory, so don't set up too many.
There is a good overview of this product at  http://www.jasons-toolbox.com/?p=99

Download from http://quizo.at.infoseek.co.jp/freeware/indexEn.html