I have now switched to Avira from Windows Defender (Windows 8). It's rated 5/5 in AV-Test lab.
Use Malwarebyte Anti-Malware as the second line of defense. Keep Hitman Pro around as well. They both are on-demand scanner. When in doubt, use them.
Scan your PC with Malwarebyte Anti-Malware and Hitman Pro.
They both are on-demand scanner. Hitman Pro is a click-to-run program and doesn't require installation.