IE sucks – that’s an indisputable fact. And all the different versions of IE suck in different ways. Therefore you need to test your web app not in only one bad IE, but in multiple ones. And to make the life of web developers even harder, Microsoft doesn’t provide a way to install multiple versions of IE on the same Windows box.
Of course, Microsoft would like you to think, that to run IE is central piece of Windows kernel and the only way to have multiple versions of it is to have multiple Windows installations... of course with all the required licenses. This is good for Microsoft, but similarly to all things that are good for Microsoft, it’s not good for you.
Luckily at least IE6 and below are available as stand-alone versions. Of course not from Microsoft, but from Evolt.org browser archive. And to make the process of installation as easy as possible, there exists a special installer from Tredosoft that allows you to install IE3, IE4.01, IE5, IE5.5 and IE6 with just a few clicks. It’s not maintained any more, but it works just fine.
Not even a restart required...