It's not exactly side-by-side but Microsoft have eased the pain, by offering a free VPC image of XP with IE6 that you can use for testing next to a full install of IE7.
Download the IE6 Virtual Image for free: http://go.microsoft.com/fwlink/?LinkId=70868
And they have now also made the VPC software free too which is very generous.
Get VPC 2004 with SP1 for free from Microsoft: http://shrinkster.com/kru
The only restriction seems to be that the image will no longer function after April 1, 2007, but they have stated that "We hope to continue to provide these images in the future as a service to web developers." so that should be ok then.
More details here