We need to test a website in both IE6 and IE7. We've had bugs appear running on actual windows machines that aren't visible under vmware (?!), so we have an actual windows laptop to do this - but only one. Is it possible to install IE6 and IE7 side-by-side in such a way that they absolutely, positively, behave exactly like they would if there was only one of them? How?
The only really reliable way I've found is to use Virtual PC and have an image with IE6 on it, Multiple IE or IETester don't always work exactly the same as the original versions
Take a look at http://tredosoft.com/Multiple_IE. Bear in mind that running both side by side is not supported by Microsoft (http://blogs.msdn.com/cwilso/archive/2006/02/01/522281.aspx) and there's no guarantee that they will work absolutely fine side by side - a Windows Update could easily break something and sometimes people find conditional comments don't work properly amongst other things. All I can suggest is you give it a go and see how you get on.
The officially sanctioned way is to use the microsoft-provided Virtual PC installation and VPC images. You don't need additional windows licenses to run these.
http://blogs.msdn.com/ie/archive/2006/11/30/ie6-and-ie7-running-on-a-single-machine.aspx
If you can publish your pages on the web, you can try http://browsershots.org/ I always use it when I have to test a new layout with almost any browser on the planet.
Virtual PC 2007 is the latest version of VPC. You should use the newer version if you're running Windows Vista. VPC 2004 isn't supported for Vista.
Download page for Virtual PC 2007 SP1
Microsoft Virtual VPC Homepage where you can find the latest version of VPC