Virus, Malware, & Spyware
The only 100% safe way to protect your computer while browsing the internet is abstinence.
But that's lame, so here are the basic protection programs you should download for your PC:
Anti-virusYou need basic anti-virus. I recommend Avast for free downloads. Microsoft Security Essentials is surprisingly good too but don't download it in conjunction with Avast (no harm done, just repetitive). I've heard good things about paying for Kaspersky. AVG and Avira are OK. Norton and McAfee are bad. Stay CLEAR away from Bitdefender; it's so invasive, it works like a virus itself. Whatever you do, pick ONE antivirus and stick to it.
Spyware/AdwareThese are the programs that detect everything from a little tracking cookie to bigger problems related to advertising on your PC. Trust me, if someone's getting paid to track your computer, get into your accounts, advertise, they'll do it. Adware and spyware happen more often than viruses on your PC because they have a clearly defined motive: money. I recommend the free version of either SUPERAntiSpyware or Lavasoft Ad-Aware, but don't do both. They (more or less) scan when you tell them to, so scan at least once every 2 weeks.
MalwareIt seems like the only one who does it best is Malwarebytes Anti-Malware. This program works in conjuction with your current anti-virus and anti-spyware. It catches whatever the others might have missed. It's non-invasive; it only runs when you tell it to. It's great as the last layer of protection.
Disclaimer: I came up with my opinions based on personal experience/opinions only. These recommendations are not fully tested.