Firefox 3 is a good browser, and is a massive improvement over 2 (which was really 1.5 anyway), I prefer the way IE handles RSS feeds and how much more streamlined the UI is (plus it works with the Tablet PC input panel).
Internet Explorer 7 is fine too, but I use 8 Beta 2 now.
As for previous remarks on security. On Windows Vista Internet Explorer is far more secure, no question. It runs sandboxed in its own security island.