It doesn’t look like it. I tried installing Safari 3.2.3 on Windows XP when Safari 4.0.3 was already installed. The installer refused to run due to a later version of Safari being installed.
So it looks like you’ll need a virtual machine to run another copy of Windows. Microsoft has made available some Virtual PC images with old versions of Internet Explorer on them, and there’s a free version of Virtual PC that runs on Windows XP Pro, so that might be a good way to go.
On the Mac, Michel Fortin has made some builds of Safari that are equivalent to older versions: see Multi-Safari.
