As web developers we have to test our websites on IE6, then IE7, and now IE8.
You can find tips and tricks to install several versions of IE on the same machine, but it may be easier and cleaner to use VPC instead.
Microsoft provides a link to download several VPC hard disk images containing pre-activated Windows XP SP2 or Windows Vista with IE6 or IE7 or IE8.
The current VPCs will expire on July 3, 2008.
Of course you'll need to install Virtual PC (free):