Emma
You do not get guarantees with anti-virus software. All bring their own problems. A lot bolis down to common sense and being cautious.
I use AVG 2012 Free. I have had 2/3 malware infestations over something like 15 years. None brought me down. I have Malwarebytes and Superantispyware sitting in the background if I have a "scare". I have had one false positive which I reported to AVG and they quickly reflected the rquired change in the Definition updates.
I do currently have a problem in that AVG have issued an SP1 update and the programme will not reflect the update. I intend to uninstall and reinstall over the weekend and expect to fix it. Although the free version is supposed to be unsupported I am getting help from AVG Technicians through their forums.
AVG does, however, have it's critics. I am not convinced as it has served me well over the years.
I have never used Avast but it is well thought of by some of the other regular contributors to these forums.
A lot of users have recently put their trust in Microsoft Security Essentials. This is another programme I have never used.
In conclusion I suggest that you stay away from Norton and McAfee. Based on my experience of troubleshooting computer problems, users of the security programmes from these two software providers seem to have more than their fair share of problems.