Microsoft had a commendable idea: build antivirus/malware protection right into the operating system. Consumers like it because they don’t have to bother with installing security software – and it’s free. Unfortunatley sometimes you really do get what you pay for, and a recent endpoint security study conducted by an independent lab indicates that Windows Defender – while definitely better than nothing – doesn’t measure up to third party commercial products.
Read more here:
http://www.informationweek.com/security/antivirus/microsoft-windows-defender-stumbles-in-m/240160249