Windows 7 Tip: 
Like most Web designers, I need to test my Web designs on as many versions of as many browsers
as I can, so that they display correctly for all my customers. Internet Explorer makes that very difficult because IE is built into the Windows OS – and so even if you use a hack, it doesn’t display the same as on a stand-alone system.
But it is possible to use a Virtual PC to run IE 6 on the same machine that you have IE 7 on natively.
While it’s not easy to install (there are 19 steps) and there are some gotchas (the hard drive image Microsoft provides expires on April 1, 2007), this is still a less expensive way to test your creations on IE 6 while taking advantage of all the changes in IE 7. How to Install Two Versions of IE (IE6 and IE7) on One Machine
alternative
1)switch to linux
2)install wine http://www.winehq.org
3)use ies4linux (4,5,6,7)
then firefox(2,3),opera and konqueror + epiphany (too many browsers? thats why we give you 16 desktops)
It sounds complicated to use virtual machines to test different versions of IE.
In Linux, it is simple to install and run multiple versions of IE concurrently. You can get an icon for each version of IE on your desktop, and you can get simple cli commands like ie5, ie5, etc, to start the browser from the command line.
You can google to find out how to install ie for your linux distribution.
On a linux host, you can run other browsers as well.
When the different versions of IE are behaving so strange, i can’t understand why the developers will waste time on making special websites for such mess. As a user, it is terrible, to visit sites that are made especially for any strange browser like IE.
Isn’t it better to concentrate on making systems that works for standard browsers?
Opera are proud of following the standards. The html layout engine for KDE and Konqueror, is copied and used by different browsers, and many powerfull organizations have now merged there work together in the Webkit: http://en.wikipedia.org/wiki/Webkit