Welcome to the site! Windows Firewall is sufficient. I recommend the following for your virus & spyware problems: AVG & Spybot search and destroy. Both programs are free with updates constantly. If you notice your PC is slow, your registry may have errors. We recommend that you scan your computer with the Hi-jack this & post it in the Virus section of this forum to be analyzed.
I agree using AVG Anti Virus. It is reliable and update regularly. However, no best anti virus can protect if you deliberately download programs from suspected sites. An attitude towards safety is the best solutions