If you're not intimidated by the command line interface, there's also a simple set of commands that will fix this problem in either Windows XP or Vista. This set of commands is presented as Steve’s “Method B” in the following thread:
http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2469173&SiteID=2
Although Steve’s methods of deleting the WMI repository were intended as a solution for the case where the Security Center fails to recognize the presence of OneCare, they should work equally well in the case where two programs are listed in the Security Center, as was demonstrated in this thread:
http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=2943165&SiteID=2
GreginMich