Activate Windows Defender

Hi all,

I recently bought an XPS 8500 with windows 8.  It came with a garbage trial version of Macaffee virus protection.  Windows 8 comes pre-installed with Windows Defender.  However you can't have both.  The Macaffee trial subscription recently expired, however it is not readily apparent how one removes macaffee or activates Windows Defender.  Each time I get to what seems like a relevant window, it only says Windows Defender cannot be used simultaneously with Macaffee, but there is no apparent way to switch from Macaffee to Windows Defender.  Please help.