If you are using Windows and have lots of ram Avast is the best free A/V. You can run a pre-boot scan to remove evil stuff that loads with Windows. No a/v or antispam ware is going to give you 100% protection. Just don't keep private stuff on your computer(bank account #s credit card #'s). Change your web browser to Opera or one of the lesser known ones. Don't have your web browser save passwords. Encrypt all your data.
Re-install Windows once a month. If someone really wants access to your computer they will find a way in. The Japanese Stock Exchange got hacked a few years back. One day someone will hack NASA and launch a nuclear weapon. Paranoid Yet!!!!