We do not live in a perfect world. Web developers have to live with the differences in the way browsers render their web pages. It is prudent to verify that the page design & functionality does not break by running the pages in all the popular browsers & their prevailing versions.
It is not possible to install several versions of IE side-by side. Thankfully you can still test on IE6, IE7 and IE8 Beta2 (on Windows XP) on the same machine using the freely downloadable Virtual PC Images.
This step by step guide shows how to run the IE8 Beta2 VPC Image on Virtual PC 2007.
Related links:
HOW TO easily verify your website design in different browsers/OSs