if IE 8 is installed "by accident" (by the Windows Update program), now the machine has IE 8. Does that mean it is safest to run 2 Virtual PCs, one with IE 6, and one with IE 7 to test our web app?
IE 8 has "Compatibility mode" for IE 7. You can click the button on the right of the address bar to revert to IE 7's rendering.
Meaning you'd only need a VM for IE6.
EDIT: Yeah, VMs are best. Compatibility Mode is good enough.
You can run multiple copies of IE on the computer. There are lots of tutorials out there, or you can use a piece of software like MultipleIE's
I recommend that you run multiple VMs with different browser versions as a way of safely testing (regardless of IE8 or not). This is what our QA team does.
Scott Hanselman posted recently on how you can install IE8 in Windows 7 using the seamless XP compatibility mode, and then you can run it along with IE6 at the same time. If you are using Windows 7, then this could be a way to go.
I find it a little odd that it's easier to run IE5, 6 and 7 next to each other on Linux (under Wine) than it is on Windows.
But hey, ho... Thinking about it, you can run Wine on Windows... That might be an option to consider. A lot lighter than a full VM.
Check out Microsoft's free SuperPreview software for comparing how pages render in IE 6,7 and 8, as well as firefox, chrome etc.
What I really like is IETester. It's an app that is easily installed (works in Vista/7, too), and you can basically have a tab for each version of IE (5.5, 6, 7, 8 and whichever is installed on your computer)
It's not quite the same, but you can get screenshots of your website in lots of different browsers using BrowserShots.
Super Preview
they said at the pdc that Super Preview in will do ie6 ie7 ie8 safari and firefox via a webservice as if it was being rendered on a mac ... its a wet dream of any webdev however its yet to come ... currently it supports ie7 and ie8 standarts mode
I'd recommend running them in a VM as others have suggested. You can get Microsoft's own VPC images from here. There are images for IE6, 7, 8 pre-installed for Windows XP SP3, as well as some Vista images too.
You can also find a more detailed how-to here for running these images under VirtualBox in Ubunutu.
Use a virtual machine. Scott Hanselman has an excellent blog post on setting up seamless virtual apps in Windows 7. The example he uses is, in fact, IE6.
If you are using Windows 7, you can use Virtual XP Mode and have IE6 there, virtualized (seamless mode). It's easy process to set it up. http://blog.sibinj.com/post/Tip-for-running-application-in-Windows-7-Win-XP-mode-%28Internet-Explorer-6%29.aspx
Virtualization is really an overkill for this, you can easily install multiple versions of IE on the same machine with "IE collection" http://finalbuilds.edskes.net/iecollection.htm
It works much better than Multiple IE mentioned above.
And if you need an application to automatically drive these (and other) browsers - check http://www.browserseal.com