When you install IE7, it automatically replaces IE6 which can be annoying if you need to use IE6 for certain sites which have restrictions on the browser version or when you want to check for cross browser compatibility while developing websites. (credit to HMMaster for googling it for me :P)
There are a couple of ways to get around this, but I found downloading the standalone IE6 to be the quickest and easiest fix to get around this. You can get it here.
If you have not installed IE7 and want to try it out but not replace IE6, you can get the IE7 standalone instead here.
via Tech-Recipes.com