If you want a paid one then ESET security essentials or Kaspersky. Some of the more 'popular' ones tend to cause problems from time to time.
If you don't have teenagers on your computer and don't download anything too suss then one of the free ones will do the trick. Microsoft Security Essentials would be a first choice because is doesn't have ads or nagware, otherwise AVG is perfectly fine.
And for backup download the free Malwarebytes and run a scan every couple of weeks, this will pick up stuff that the AV programs wont, like potentially unwanted programs, Adware, Malware etc.
