Since the portable versions of Internet Explorer are no longer supported (and never fully worked) I\'m trying to find a way to test sites in a number of different browsers.<
Like Flakron Bytyqi suggested, the easiest way is to use IETester or similar software. Using IETester for a year, I never saw any difference between its IE6 implementation and the rendering by real IE6.
Now, having multiple virtual machines can be a good idea, since:
Since installing three VM and buying three Windows licenses is not funny, I personally suggest using IETester, as said before, or using services of companies like Litmus (but it's true, you can't test functionality).
If running Windows, for Internet Explorer try IETester or IE Collection. They contain all versions of Internet Explorer.
The best way is to actually install these browsers, and really use them to test. You may use virtual machines in some cases (when you want to test Internet Explorer 6 on a Mac, for example).
Have you tried Adobe BrowserLab?
But for functional testing I'm afraid you will have to do it in browsers. If you are using Linux, you can have a Windows XP installation with (Internet Explorer 8, Safari, and Chrome) on VirtualBox on a Linux machine (Internet Explorer 8 has an option to behave like Internet Explorer 7). And do Internet Explorer 6 testing on Wine in Linux. Checkout PlayOnLinux for running Internet Explorer 6 on Linux.