Avast home edition is free and effective, it protects your PC from infected webpages, scans your downloads and emails for viruses and has built in anti-rootkit and anti-spyware, plus it gives you the option to run a boot scan, there are very few anti-virus's that can do this paid or otherwise and this is free. I use Avast along side Comodo firewall both are free, light on memory and have been independently tested and proven to be more effective then Norton or Mcafee.