If you look at this article
http://blogs.techrepublic.com.com/wi...ontent;leftCol and scroll to about half way down is that the pop up you`re getting "Windows has blocked some start up programs" ?
p.s. I agree with Ben B, if you have a decent Anti-Virus/Anti-Spyware then imo windows defender just takes resources and should be disabled.
If you are happy that your current security software will do the job to disable windows defender got to Start->Type in services->click the entry services with the 2 cogs next to it->a new window like the picture will appear, scroll down to "Windows Defender" and double click it->In the new window select "Disabled" from the drop-down->Click Apply then Stop.
Windows Defender will now not start on next re-boot.