Unfortunately, there are no security solution that will prevent unknown (new) malicious code from getting on your computer. Not even the FBI can depend on any one solution.
That is why having a good understanding of basic cyber security, good online practices, and various security tools available is the best practice.
You can have the freebie version of MalwareBytes installed with your McAfee for added protection. If you find issues having both, then uninstall Malwarebytes as it is not a full blown virus protection, and only use it when you want to run a full scan on your system.
Bottom line.. there are no guarantess in life and there aren't any in the virtual world either. You are your best protection.
MYSTERY SOLVED! The redirections happen because I am using Roboform as my home page.
Problem was that although I have set Google as my default search provider in IE9, when I search from the search field of Roboform screen, I get Yahoo results.
When I search from the Google toolbar (not being part of my Roboform home page) I get Google results.
Turns out that the redirection has nothing to do with malware, it is a new Roboform policy. The explanation they sent me is:
"We recently changed our search engine on the Start Page from Google to Yahoo at Google's request. Google's new rules prohibit software vendors like ourselves from leveraging the Google search engine. We apologize for the inconvenience. We are still trying to work with Google, but it is mostly out of our hands at this point."
Thank you to Ex_Brit and sol, the Stinger and MalwareBytes scans came up clean, by the way.