If you currently have Office 2003 (or any Office 2003 component such as Visio or Publisher) installed you’ll run into some problems if you upgrade to Office 2007 Beta 2*. The Office 2003 components will not be completely removed and you may have issues such as returning shortcuts or Outlook add-ins trying to reinstall eventually bringing Office 2007 to a grinding halt. If this happens, check out the “Windows Installer CleanUp Utility” for removing all vestiges of Office 2003. You may then have to perform a repair of the Office 2007 applications. See KB290301 for more information. Probably best to remove an earlier version of Office before installing Office 2007.
(* It’s probably mentioned in the release notes somewhere but why waste time reading release notes when you want to check out the new Outlook?)