Free, powerful, and easy-to-use antivirus software from Malwarebytes, a reputable virus and malware protection company.
Windows 8 / Windows 7 / Windows Vista / Windows XP
Malwarebytes has been in the antivirus scene for a long time, and their Anti-malware Pro suite has one job: eliminate all current infections, and prevent all future infections.
There is a lot of confusion about what Malwarebytes Anti-malware is actually supposed to do. Historically, it has only done reactive scans but has not prevented viruses from actually executing. However, this changed with the release of their premium "Pro" version (though it should be noted that it's still cheaper than most other solutions on the market). The Pro version does proactive scanning, preventing users from running files that may be infected, while still doing the appropriate background, registry, and heuristic scans.
Anti-malware Pro does its job well. It can be configured to automatically retrieve virus definitions, do scans on regular intervals, and take care of viruses currently on your computer. They also offer services that let you run it on already infected computers that may contain a virus preventing anti-virus tools from running. A big complaint about Malwarebytes' software in general used to be that the interface was clunky and hard to use. Within the past year it was revamped, making this a highly recommendable product. Just a cursory scan will indicate that it has a very high detection rate, and there's no reason to not give it a try.
All-in-all, there's no reason to not try Malwarebytes Anti-malware Pro. The company behind it is highly reputable, and it's a solid piece of software that has endured through times when most of its competitors got cut out of the market. It's good for anyone from the most novice user who just needs an easy setup to the poweruser who needs access to advanced scans and options.