antimalwarebytes seems to be the most popular. There are several good free ones, and none are really "better" as they all have strengths and weaknesses.
1st thing is to make certain to completely remove all traces of the previous AV softwares. I use Revo (free) and then run the specific tool to remove the software. Norton AV Remover, AVG Remover, etc... Do this before installing the next one.
Here's my standard lecture:
There are 3 components for computer security, and the AV software is only the 3rd one. No software can keep a computer safe when the User is stupid and/or determined to get infected. Browser behavior is the primary reason computers get infected.
1) Windows Updates are current. WU closes security holes as they are discovered.
2) Firewall
3) AV software. Better to make sure the malware doesn't get onboard by controlling browser behavior, closing security holes and having a good firewall in place. AV software works after it's too late.
Meaning, don't have too much faith in your AV software. People that do get infected a lot more frequently than people that do not.