Hey there,

Personally I'd recommend Avast Antivirus..it's free like avg but I found it to be much more effective. It has saved my laptop on numerous occasions.. and it's virus database updates itself every day which is a major plus. You can download it from this link: http://www.avast.com/eng/download-avast-home.html
PS. make sure you download the home edition as there is also a professional one but you have to pay for it.