Malwarebytes Anti-malware ain't free any more. It installs as a trial. I had a case where this didn't even detect a malware variant.

Combofix can be risky. I think Bleepingcomputers recommend it as a last resort because it may remove the malware but it may also remove other things that aren't malware.

I like Spybot. Use it mostly as a verification.

Avast Free is garbage. Fully up to date PC and Avast and it couldn't detect the fake "Anti-virus XP" on 3 system. After that I don't recommend it.

MSE is lightweight and does a decent job but should automatically update the PC at bootup. In one malware exaple it removed the "main" malware file but couldn't pick up the one that triggered it. Actually Malwarebytes couldn't detect it either.