Perform the following checks:
Verify that you have removed all other real-time antimalware products that were ever installed on your PC, including free/trial product that were installed when the PC was purchased (Norton, McAfee, TM, AVG, OneCare, Malwarebytes (paid), etc). Then, use the cleanup tools here:
If you just updated Adobe Flash, you may have inadvertently installed McAfee Security Scanner. Verify that it is not present.
Verify that Windows Defender is uninstalled (if XP), or Disabled (if Vista/W7).
Verify that you are up-to-date with patches and service packs.
Verify that your PC clock is correctly set.
Verify that you are using version 1963 of MSE (as of June 28 2010). For more information, review http://support.microsoft.com/kb/2254596 .
Remove Zone Alarm if you have it and use the Windows Firewall.
Verify that the Windows Firewall is on, and set to defaults.
Verify that you are not using a proxy server.
Verify that you have “install updates automatically” selected in Windows Update (even though the MSE definition updates are optional).
In IE, reset all security zones to default level.
Then, restart your PC.
If the problem persists, submit a support case to Microsoft: https://support.microsoftsecurityessentials.com/?scrx=1