If you need to test a website in a different version of Internet Explorer from the one you have installed, Microsoft provides VirtualPC images for Internet Explorer 6.0 and later, which can be downloaded here. For older versions of Internet Explorer (3.0, 4.01, 5.01, and 5.5), there's a nifty installer at this site, which will install them all for you. I'd recommend installing that on top of the virtual machine for 6.0 (rather than your regular environment) given the likelihood of unpatched security holes in old versions of Internet Explorer.