isutton,
All Windows Vista versions are compatible with OneCare. So there is no needed application that needs to be installed first for onecare to work on that platform.
Are you installing onecare from a CD? If so, try to install onecare directly from its homepage. http://onecare.live.com
Install the trial version then activate with your subscription.
If that fails, please contact support.
How to reach support (FAQ) -
http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2