Do you have more than one antivirus running, if so the new or other antivirus will always try to disable security center being that with the expection of MBAM you dont want two antiviruses running, they will fight eachother.
After you login
Press Windows key + R
Scroll down to Windows Defender
Select it make sure it is running and it is status is set to Automatic.
If not fixed AND if you have another antivirus you can either delete the 3rd party antivirus, or in this same services.msc turn off windows defender, which most antiviruses will do for you be default.
this is from www.answers.microsoft.com
But BEFORE you do any of this please backup your files and make a recovery disc so IF something happens you can get back to at least where you are now. If you have any questions please ask me before you post. I will check back around noon CST USA.
Try the above stuff first and reboot, then do the below, but BACKUP first here is a guide from Microsoft.com
Consider changing the way Security Center alerts to disable the warning.
There have been some recent reports of this situation. It may be due to a miscommunication between MSE and the WMI database. It may also be due to undetected malware.
Some people report that opening MSE, performing an update check and turning off and back on again the MSE real time protection resolves the issue temporarily or permanently.
If the WMI repository is broken, the following may help:
We've seen a number of reports here of this behavior following the recent upgrade of MSE.
I suspect that there are multiple causes.
Some have reported that uninstalling and reinstalling MSE resolves it.
Others have said that forcing an update or running a scan with MSE each time it happens solves it after a few days.
Others have reported that turning real time protection of and back on has resolved it.
And still others indicate nothing fixes it.
The problem can also be due to a broken WMI repository (see below) and perhaps a resource issue -- that is, the PC is low on memory or running slowly due to many programs and processes starting with Windows causing MSE and the Alert Center to not communicate in a timely manner after which it gives up (either reporting status to WMI, or WMI querying the MSE status). Try a clean boot or disabling excess startup programs to see if that's your case.
Here are procedures to correct the WMI repository that causes the Windows Security Center to report that no antivirus, antispyware, or firewall is active, yet MSE is actively protecting your PC and in green status.
When running this in Vista7, you must run the procedures as an administrator, elevated.
Scroll down to Windows Management Instrumentation and double-click it.
Now click on the "Pause" button. Leave that window open and double-click
My Computer. Navigate to %systemroot%\Windows\System32\wbem (where
%systemroot% is the drive where XP is installed). Delete the Repository
folder and *only* the Repository folder. Now go back to the WMI service
window you left open and restart the service.
This will rebuild the Repository and hopefully straighten out the
incorrect entries for all your duplicates.
In order to see the Windows files, you may need to unhide them:
Make sure you are able to see all hidden files and extensions (View tab
in Folder Options).
Check "Display the contents of system folders".
Check "Show hidden files and folders".
Uncheck "Hide protected operating system files" and click "OK" to the
Method B from MVP Torgeir Bakken (more elegant)
Open a command window (Start/Run --> cmd.exe) and run the following commands:
net stop winmgmt
cd /d %windir%\system32\wbem
ren repository repository.old
(or delete it using the command "rd /s repository" instead of the ren command)
net start winmgmt
It may take a minute or so to complete while WMI rebuilds the database.
Alternatively, see this blog post: http://ashwinrayaprolu.wordpress.com/2011/05/13/repairing-fixing-wmi-repository/
If you suspect an infection:
You can start here: https://consumersecuritysupport.microsoft.com/ (which will lead to the paid support options of http://www.answerdesk.com if you are in the US)
In other regions not served by the link above, go here: http://Support.microsoft.com/security and go to the “assisted support” or contact us menu.
This web site - http://www.bleepingcomputer.com - contains details for many of the common infections, often immediately after they began to appear in the wild, and instructions are provided for how to remove the infections using their malware removal guides. They also have forums where you can seek help from people who specialize in malware removal.
This may also be helpful - How to get rid of malware:
Try Hitman Pro Trial Version: http://www.surfright.nl/en/hitmanpro This program may be run from a flash drive. You may need to run it in Safe Mode or Safe Mode With Networking.
Try TDSS Killer: http://www.bleepingcomputer.com/virus-removal/remove-tdss-tdl3-alureon-rootkit-using-tdsskiller TDSS Killer may be run from a flash drive. You may need to run it in Safe Mode or Safe Mode With Networking.