I was using Windows Security Essentials, and it has not been removed.
running multiple AV can / will create all kind of mysterious windows errors and false positive detection

Never install two antivirus (see reply from quietman7)
http://www.bleepingcomputer.com/forums/index.php?s=7c8217673a726b92cfc91ecfd4294a29&showtopic=260844&view=findpost&p=1441638

it is also recomended to run a removal tool and reboot to clear all leftovers
can be found here http://thewebatom.net/uninstallers/security-software/