While developing a project I needed to test using different versions of IE. It was difficult finding computers with older versions of IE on them to use for testing. I went to friends and a few old boxes but I was never able to test with IE 5.01. After reading an article at Chicago Web Design, I found a way to install different versions of IE on the same PC and run them concurrently. Here's a screen shot of IE 5, 5.5, and 6 running concurrently on one PC. You can tell that it is running each idividual version of IE because there are specific CSS bugs that only each version of the browser will render.
This seems like it would be a great tool for a developer to validate that a Web project works with specific versions of IE. Ryan Parman has created downloadable files with just the files needed to create multiple versions of IE. Just download the zip files. This site also has some very good information on the process. I created a directory called "ieold" and extracted the zip files into it. The zip files created an IE501SP2_NT and an IE555SP2_NT directory. Just run IEXPLORE.EXE in each directory and you get the respective versions. There are some caveats, so read the whole article.
It works great! I'm now able to test new code with older versions of IE on one PC. If you're brave, it looks like you can even run IE 3.0 and 4.01.