Options 1 : IE Tester
IETester is a free WebBrowser that allows you to have the rendering and javascript engines of IE9 preview, IE8, IE7 IE 6 and IE5.5 on Windows 7, Vista and XP, as well as the installed IE in the same process
http://www.my-debugbar.com/wiki/IETester/HomePage
Option2 : Multiple IE Installer
Install Multiple IE installer
Run IE6 , IE7 and IE8 side by on using Windows 7 RTM. Here is a nice blog post on how to do this
run IE6 , IE7 and IE8 .
Multiple IE installer is no longer supported and maintained. So take you chance.
Option 3
Internet Explorer Application Compatibility VPC Image
This is VPC Hard Disk Images for testing websites with different Internet Explorer versions on Windows XP and Windows Vista. You will download different virtual images on your machine and need to install them.
I have not tried it but will trust it as it is from Microsoft.
Option 4
Download and install Virtual Box.
This is free vitualization software for enterprise and home use.
Option 5
Read more: Skill Guru