I like to run second opinion scanners from time to time on computer systems to make sure that no malware or other unwanted software has slipped by the primary defenses running on those PCs.
While that is the case, I don't have have a preference for a tool that I run in this case but use different programs.
There are programs that I value a lot, like Malwarebytes Anti-Malware or Hitman Pro, but it never hurts to run other programs as well on the system.
Trend Micro Anti-Threat Toolkit is a free portable program for recent versions of the Windows operating system.