There could be a couple of things going on there, first and foremost is
that WLOC disables the standalone Defender because the Defender engine is
built into OneCare so why run two copies of it.
http://forums.microsoft.com/WindowsOneCare/ShowPost.aspx?PostID=1105575&SiteID=2
Then If you think that the Security Center is not reporting to you
accurately, and you're on Vista...
Run a cmd prompt as administrator.
Enter the following command: winmgmt /verifyrepository
which checks the integrity of the WMI database and let us know the results.
If your on XP the above command won't work however.