I've had pretty good success with
Avast free anti-virus software. Equally, I also run Microsoft's malware and virus removal tool, which is also free. It does a pretty thorough job. Just a few days ago, it caught and removed a serious and medium threat on my computer. Just go to their site to download the software applicable to the system you are running (Windows 8, 7, XP, etc). They have a short and complete scan. The complete scan takes a lot longer, but it's a good idea to run it if you think your computer might be infected.