One of the downfalls of running Internet Explorer on Windows remains the fact that it's darn near impossible to have more than one version of the browser on a single computer/install. I know several folks that have older PCs for the very reason of being able to run an older version of IE on one PC and a newer version on a second PC, or running multiple boots with a different install on each. Pretty ridiculous when you think about it.
I read a note over at Ryan's 6foot6 Design site that mentioned that Joe over at Insert Title Web Design (great name, btw) has mastered a way to install several versions of Internet Explorer on Windows. For example, you could have IE 6, IE 5.5, IE 5.01, IE 4, and IE 3 running on the same Windows install. It's the holy grail for designers.
If you're trying to install multiple versions (or legacy versions) of Internet Explorer on Windows, check out Joe's instructions and notes.