Windows Live OneCare Beta 1.5 is available and supports Vista RC1 and greater, and can be downloaded and used free of charge, is available now on http://ideas.live.com.
From the FAQs:
What operating systems are supported?
Windows XP SP2 or Windows Vista (build 5500 or greater; recommended build is 5600 aka RC1)
Can I use Windows Live OneCare with Windows XP or Vista x64 versions?
Not at this time.
Tags: customer satisfaction, Onecare, Microsoft, Windows Vista, Windows, Vista, Vista upgrade