What you could try doing first is disable Microsoft AntiSpyware from the
Notification Area (right click the icon and click Shutdown Microsoft
AntiSpyware). Navigate to c:\program files\Microsoft AntiSpyware and delete
the error log. Go to Ad\Remove Programs (Control Panel) and the uninstall
Microsoft AntiSpware restart the system then install back MSAS.
The reason is not known some say it has something to do with limited
mulitple account scanning and fast user switching not being supported .