What do you need to protect your PC?
Anti-Virus- You need an Anti-Virus to protect your computer against viruses, trojans, and worms. These programs can reek havoc on an unprotected computer.
A good anti-virus will provide, on demand scanning, scheduled scanning, always on background scanning that scans all new files and all new changes to programs for virus codes. Also a good anti-virus should scan for potential viruses by detecting characteristics of viruses coding and behavior.
Recommended Anti-viruses: McAfee, Norton, AVG, Avast

Firewall- A firewall protects against unsolicited connections to and from the internet. It keeps hackers out and protects against malicious connections to the internet so even if a malicious program gets on to your computer, it wont be able to "phone home." This helps neutralize trojans and spyware malicious effects on your computer. This does not mean you should not use an anti-spyware software since spyware sometimes insert their programing into legitimate programs and hijack them to perform their designated operations
A good Firewall should block all unsolicited incoming connection attempts and control all outbound transmissions.
Recommended Firewall: McAfee, Norton, ZoneAlarm

Anti-Spyware- Anti-spyware protects your computer against programs that spy on your computer and make pop-up advertising and ads on your computers as well as hijack your browser and redirect your computer to malicious sites
There is no silver bullet in spyware prevention but some products are showing up as industry leaders. Good products should do the same as good anti-virus. They should offer scheduled scanning, on demand scanning and background protection.
Recommended: Spyware Doctor, Spy Sweeper, Microsoft Antispyware, Ad-Aware, McAfee