There seems to be quite a few issues when trying to install the beta on systems that have or have had a previous version of Office on them. The only way I got it to install on my system was using a program called RipOutOffice2007, then going in and deleting all of the Office folders from my Program Files directories.
If you want to keep your 2007 install, then this is not a method you want to use. Hopefully someone can give you some advice on getting them running side by side.