Are you using OneCare and received a message that "antivirus and antispyware could not start" and this may have happened after a OneCare update to 1.6? if so, see this post - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=1695761&SiteID=2
If that doesn't help, support - http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=1183038&SiteID=2
If you're not talking about OneCare, then you're off topic.
-steve