If you are a web developer, then you will definitely need to test how your web site looks on the new IE7 as well as the old IE6. However IE7 is installed on top of IE6, so there is a problem here.
At the end of last month, Microsoft has offered a not-so-easy solution to this problem.
For Windows XP/2003 install Virtual PC 2004, for Windows Vista install Virtual PC 2007 (beta), and then download a ~500 MB Virtual PC image containing a pre-activated Windows XP SP2, IE6 and the IE7 Readiness Toolkit. After all this, the image will expire on the April 1st, 2007.
If you like this idea, then get the MS Virtual PC image here.
A completely different approach is to use the Multiple IE installer, and make sure you read this article as well.
This way you can even have all IE versions (3.0-7.0) on the same Windows installation.
Some users have been reporting that the Multiple IE program could be a buggy solution.
There are interesting comments on this topic here and here.
For the VMware customers, I would suggest to import the MS Virtual PC image using the VMware Virtual Machine Importer. Newer image versions are supposed to be released by Microsoft, as the current one is set to expire as I said before. However you could always “play” with the VMware clock settings, and maybe avoid the time-bomb!