You said to buy, not a freebie. I really rate FSecure internet security package. Just renewed mine for the third time, well actually this time bought it from Amazon for under £20, you get the disc and a 3 PC license. Use FSecure and NOTHING else as recommended. It has it's own firewall etc so turn Windows one off. Never had any malware problems.
I use ESET Smart Security and they recommended that I use SUPERAntispyware too as at the moment no spyware program can detect all spyware and malware. They both work well together and don't slow down the computer.
I agree with mooley I've used F-Secure for several years now with no problems. I tried Kaspersky last year and had to get a refund as the firewall was driving me bonkers! Every few minutes it would pop up with warnings, none of which where truly important. I would however try them again as I had no problem getting a refund and their tech support where great, they actually changed the default setting in later releases as a result of the problems I, and presumably others where having.