Integral beat me to it, but yes: Presuming you don't have an upgrade disc, but rather have the full "OEM" copy (i.e. one that can install Windows from scratch), this is the way to do it. It's called a "clean install", and there are directions out on the web that you can use.
Keep in mind that since you're downgrading, you're most likely going to want to erase your hard drive first (that's where the "clean" in "clean install" comes from). So you'll want to back up the stuff you want to keep. You can do this manually, or if you have room for a fair amount of storage somewhere (blank DVD, external hard drive, whatever), you can use
Vista's "Easy Transfer" wizard.
And you'll also want to make sure you have the proper "drivers" for your system. Otherwise, things like your video card, sound card, network card, etc. won't function, and you'll be in a heck of a worse situation than you are now. If you're lucky, your computer's manufacturer will have given you a disc or several discs that contain those. If you're slightly unlucky, you'll have to hit the web and download the right ones for your computer. If you're seriously unlucky, you won't be able to locate those drivers. In that case, you might want to reconsider getting rid of Vista just for now. So seriously, make sure you've got those drivers
first, because otherwise you'll screw yourself and make the process waaaaay harder than it needs to be.
Beta? The Release Candidate is out already. Which reminds me, I need to wipe my own install of the beta and go to the RC... damn, I used to actually have time to geek out with new stuff like this in the past. What's happened to my life??