Go to add/remove programs in Control Panel (Programs and Features in Vista) and select Norton Live Update (or Symantec Live Update) from the list and remove.
If it isn't there, contact OneCare support for help -
How to reach support - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2421771&SiteID=2
If it fails to validate your subscription, select the option that you are using a trial or beta copy and you can proceed to email support without validation once you've signed in.
-steve