While testing website code I often get to the point of using multiple computers and multiple systems and multiple VM Wares.
I have some old CD-Roms with old browsers, but let’s get real, I don’t carry them around with me all the time, so I was out searching for some older IE versions and stumble across this collection of IE 5.5 standalone and older versions.
You also could consider using browsercam.com, browserhots.org or browserpool.de — at least for a quick check how the website does look on different OSs/browsers.
Have a look at NetRenderer. This is a new online service that renders your webpage in Internet Explorer Version 5.5, 6 or 7. Most interesting is the IE7 IE6 difference and overlay viewer, that lets you instantly detect any rendering differences. It´s very fast too…