I'd recommend to install to the default directory. Each beta version will default to its own individual directory, such as "C:\Program Files\MoI beta Sep-26-2006".

So by default there is no overwriting happening and you can have multiple beta versions installed at once with no problem.

It is probably a good idea to keep the old version around for a little while, just in case something that you needed to use got broken in the new beta (hopefully this won't happen, but it is a possibility). After you've used the new one enough that you feel you don't need the old one anymore, you can uninstall it. There is a shortcut for uninstalling off the Start menu / All programs entry for the beta.

- Michael