IE7 & IE6 on the same box?
As most web developers out there who're designing for modern browser technologies will have no qualms ranting about, Internet Explorer is problematic. IE6 has a number of rendering quirks that can be absolutely maddening to fix. There's a reason why a number of clever individuals have come up with a number of clever hacks to make IE6 play nice. Then along came IE7, with the promise that IE6's bungled implementation of CSS would be fixed....including those clever hacks.
This puts us developer-types in an awkward position. We can't ignore IE6 once IE7 ships because of the scary market penetration the older BigBlueE already has. It'll take at least a couple of years after Vista is released into the wild before we can even begin to feel okay ignoring IE6's quirks. Thus, we'll need to be testing our code in not only our happy, modern browsers, but also IE7 (I'm pretty sure the folks in Redmond haven't quite gotten their browser up to where Gecko and whatnot are/will-be-when-Vista-ships), and IE6 (can't ignore those users who can't/won't/don't-know-to update).
Problem: IE6 and IE7 can't be installed simultaneously on the same OS. Really. It just can't happen.
Solution: If you can't afford (or don't want to) maintain a seperate WinXP box just so you can use IE6 for testing, I've heard people people recommending the usual Virtual Machine options.
Better Solution: Have a neutered copy of IE7 available for testing purposes. Find out how to do this at the GoogleSystem Blog. Pretty simple, pretty easy, and I definitely got it working on my Windows Box in less than five minutes.