As much as I wish people would move off their old versions of IE and Firefox as fast as possible, it's rather hard to induce this. Microsoft has tried hard to move people to IE7 by making it a Windows Update. However, the simple truth is that when developing web sites, you must support many different browsers, including those old, terrible versions that you wished had never been released.
There is a small problem with this: finding these older versions of IE is rather difficult. Oh they are out there somewhere, but it's not easy and obvious. I usually resort to downloading some version of IE 5.5 off of a sketchy website, and then proceed to do multiple AV scans of the file I've downloaded.
And what should come to the rescue...Linux, naturally! I recently found a project called IEs4Linux that installs IE 5, 5.5, and 6 off of Wine. It's meant to be used for web designers to test site compatibility in older IE versions. Personally, it will be useful for my own web testing -- and now I don't have to use Windows to test IE anymore.