Un-install all the McAfee files off your computer if you dont want it anymore and see if you can switch on Windows defender then. If not, either buy McAfee or get some other anti-virus program or leave your compy unprotected, but yeah....

I use McAfee, and while its actually a pretty good program, its a bully to all other anti-virus programs. Its not compatible with anything else, really, thats why it switched off Windows Defender.