There are two options for you to install Vista SP1, one is the legal way and the other illegal way. The illegal way involves downloading the service pack from torrents, which I am not going to tell you about here. Rather let's focus on the way Microsoft wants its beta testers to download the Service Pack .
The trick is pretty much straightforward, so grab yourself a coffee as you do this. This won't take much time.
What you can do is legally install SP1 and apply the Windows update hack by downloading it via Windows Update. Here's how you do it:
Download this file (SP1Beta_Hack.cmd - Right Click and Save as to Download) and run it with the Administrator Privileges. You should see a successful message displayed on screen in Command Prompt. Now run Windows Update and Check for updates. It should show you the KB935509 hotfix. Download it and restart your computer. Run Windows Update for the second time and Check for updates. This time around you should find another update called KB937287. After downloading the second update, restart and run Windows Update for the third time. This time you'll download KB938371 hotfix.