Today having a good security Anti virus software is a must, Ive used AVAST free version for three years, and I am a power internet user, Im online ALLOT and Avast has protected my computer perfectly, I recently Bought there paid version since Im now banking from the road, again its a great system, the free version for HOME users works great for casual surfers etc.....making sure Windows is updated, Use Defender often, do a weekly Virus Quick scan, and a monthly Virus full scan.....
Other great programs that are available as Freeware,
Hitman Pro...there is a free scanner/cleaner and a paid version, I do know many IT professionals rely on Hitman Pro when issues arise.
Crap Cleaner....a free cleaner that cleans caches, garbage and unused cookies and files, used daily or weekly will improve the speed of your computer to like new.
Spybot....this removes the little Bots that are placed on your computer by programs, it cleans many Malware items and Immunizes your Browser to prevent reinfection.
Malware Bytes, a free version for home users is available at Gizmos or CNET, As are most of these programs, it scans and removes Malware and is VERY effective.