Windows Defender was initially based on GIANT AntiSpyware, formerly developed by GIANT Company Software, Inc. The company's acquisition was announced by Microsoft on December 16, 2004.
Windows Defender didasarkan pada perangkat GIANT AntiSpyware (anti-pengintaian GIANT), yang mulanya dibuat oleh GIANT Company Software, Inc. Akuisisi perusahaan ini oleh Microsoft diumumkan pada tanggal 16 Desember 2004.