When I was researching ways to run IE6 for browser testing purposes on my Windows 7 machine (other than with IE Tester, which is buggy, or Multiple IE, which I don’t think even runs on Windows 7), I came across XP Mode.
It’s simple to install, and it comes preinstalled with IE6. You don’t even need a serial number for XP. Just follow the steps on the page (they’re surprisingly straightforward) and you’re golden. The only thing that was even slightly tricky was configuring BIOS if your hardware virtualization isn’t turned on, but I managed with no problems, and I never do any configuration.
I never thought I’d say this, but “Thanks Microsoft for this helpful development tool.”
Installation page: http://www.microsoft.com/windows/virtual-pc/download.aspx
BIOS configuration instructions: http://www.microsoft.com/windows/virtual-pc/support/configure-bios.aspx