Installing Visual Studio 2005 SP1 Beta on Vista RC1 sounds like a common task until you figure out the UAC - User access control, and the enhanced security functionality in Vista.
You may find that your install suddenly just stops in the middle or does not work at all.
The solution is quite simple:
You need to run the Setup as an Administrator on the machine (shift right click, "run as"), and from then on, you need to also run VS 2005 as Administrator on the machine (you can define a shortcut on the desktop that has the "run as" fixed on it as admin).
here are some more links to help you with more problems SP1 may cause:
Thanks to Corrado for posting this on one of the mailing lists I frequent.