I too had WinDefender and could not tell if it was the real one of this fake one that I found out about via google search:
http://www.enigmasoft...defender2008-removal/
It was a pain finally removing it. I had to use malwarebytes and spyhunter. Both found the fake one but after rebooting it came back. Somehow I was able remove it fully in Safe Mode. '
To keep from getting it again is almost impossible. I would just keep your antimalware program updated and do not trust your antivirus program to protect you from it. Also, stay away from free porn sites, those are filled with Trojans and stuff that install these fake security programs.