If using Windows 8.1 I will only recommend that you use Windows default security features, third-party security products cause a lot of problems on Windows 8.1.
On Windows 7 or Vista:
1. Avast Free with Windows Firewall.
2. Avira Free with Windows Firewall.
3. AVG Free with Windows Firewall.
4. Panda Cloud with Windows Firewall.
5. Microsoft Security Essentials with Windows Firewall.
It is very important to only download files from safe and utilize UAC prompts (never approve anything unless you know it is safe), since no AV will be able to detect every single threat.
I don't recommend Qihoo 360 since its multiple AV engines detects far too many false positives and it is still fairly new, has bugs and no English support.
I don't recommend paid AV's because you can get infected just as easily using them, no proof they offer any better protection.
According to my experience of dealing with my customers paid AV didn't offer them any better protection.
If you don't know how to download files safely then learn use a "Limited User" account until you get it down.
No need to add a bloat load of security product to drag down your system and create problems.
Enjoy!!
