Pick the top rated one, and get it. This is how you decide, not by what peeps suggest,, check the reviews out first. Free superantispyware is fine, for anti spyware, although of course the paid is always going to be better.
I recommend Avast home edition. There is a free version, but you can pay to upgrade. I have used McAfee and Norton and nothing but problems. I have had Avast for two years, trouble free. And use friendly. I use it in conjunction with Zone Alarm firewall. If you are set on paying, please check out the Avast web site.