I stumbled upon this marvelous creation, allowing you to run both Internet Explorer 6 (IE6), Internet Explorer 7 (IE7), Internet Explorer 8 (IE8), Mozilla Firefox 2 (FF2), Mozilla Firefox 3 (FF3), Google’s Chrome (1), Opera & Apple’s Safari on one computer. It requires you to visit a website, from where you can launch these applications. It’s brilliantly simple.
Check it out: http://www.xenocode.com/browsers/
It sure beats having to run a Virtual PC with several seperate images (XP + IE6, XP + IE7, XP + IE8). If you want to try out different operating systems (XP or Vista), you should still revert to virtualising your OS (try Virtual PC + premade OS images by Microsoft). Now to find a solution for running IE5.5 alongside it! ;-)