I used to use and recommend Microsoft Security Essentials (which has bigbass1997 mentioned is now Windows Defender as part of Windows 8 and 8.1).
However, its effectiveness has degraded in recent antivirus tests so I have switched all my computers (and family's computers) to Avast Free. Register your username and you get free and excellent antivirus for a year. Make sure to set it to Silent/Gaming mode so that annoying voice doesn't interfere.
I would definitely recommend against
Norton as its reputation is to be slow and bloated, however I do not know the performance impact of the latest Norton 360 to judge.
If you are on a Windows machine and not
using an antivirus, you are, ehmm, in the most kind terms, an idiot