I bought the SAS lifttime license for USD9.95 and personally regret it. Luckily for me it is mere under USD10, if not I will cry :'(
I can't comment on the effectiveness of SAS as I never come across any virus as yet. I can only comment on these. Despite it has been updated several version since I bought (4.2x to current 4.38), I found these SUPERAntiConveniences :
1) the virus data update took very long (sometimes upto 15 minutes)
2) the version update (need administrator right in Vista) took my lifttime (sometimes after an hour then ... restart the update again)
3) the scanning took too long and make my Acer laptop very "hot", Acer is cheap but easy to heat I knew that.
I would recond a free version only if you would own it. Never buy it.