I have just updated my McAfee anti-virus for another year at a cost of £29.99. I felt it was worth it. I have also noticed that you can get from PC World over the next few day the McAfee Internet security suite version 7 for the same price including quickclean.
If you do decide to go for a free version, I would suggest either AVG or Avast. You would have to uninstall McAfee of course before installng any other anti-virus software.