I’ve been running IE 7 beta 2 in standalone mode (so it wouldn’t override my real IE installation) for a while. However, when beta 3 came there were some issues going to beta 3 because it needed to write some values to the registry and didn’t assume your default settings.
I ran across Tredosoft’s entry on how to get beta 3 working. The really nice thing is that they packaged a nice installer to do everything for you! So, if you need to test your website in IE 7 (there are some funky things going on with style rendering), I definitely recommend checking this out!