I used to use avast free which I was quite happy with, and it is still on my sons PC but I thought I would try adaware (I know I used to just think it was a spyware cleaner)
http://www.lavasoft.com/ ...anyway I tried the free version, then after a few times of loading it up it gave me a message saying 12 months of Pro for £7.29 so I got it as I had a bit of money in my paypal, then when I bought it I got another message saying 'like us on facebook for another 3 months free' lol
15 months ad-aware pro for £7.29 is alright to me!
I also use:
McAfee site advisor browser plugin - free
McAfee security scan plus - free (bit like the old McAfee stinger)
Housecall is also a good free online scanner