I've been onto all kinds of sites, reading about people who cannot start windows defender, and cannot start it manually. Relax! 
First Windows defender is only back up antivirus software graciously adapted into Vista, really its only for those who are not running any antivirus software at all. Sooo, if your running any other antivirus software you should disable defender anyway. Multiple antivirus packages running together is not a good thing. They litterally fight against each other, so only use one.
Secondly, defender will not run, and you cannot start it manually if you are running microsoft security essentials. Essentials will automatically turn off defender. Don't freak out, this is supposed to happen. If you really want defender to run, you must uninstall essentials. Although, I highly recommend essentials, Norton 360 is also great.
First Windows defender is only back up antivirus software graciously adapted into Vista, really its only for those who are not running any antivirus software at all. Sooo, if your running any other antivirus software you should disable defender anyway. Multiple antivirus packages running together is not a good thing. They litterally fight against each other, so only use one.
Secondly, defender will not run, and you cannot start it manually if you are running microsoft security essentials. Essentials will automatically turn off defender. Don't freak out, this is supposed to happen. If you really want defender to run, you must uninstall essentials. Although, I highly recommend essentials, Norton 360 is also great.