Viruses are no longer the biggest problem on the net, trojans are. You need a firewall, an anti virus, an anti trojan, an anti adware and an anti spyware and you can get all this free for home use from reputable companies.
I'd suggest Avast!, Ewido, Spybot, Ad-Aware and Ccleaner plus a firewall and Winpatrol.
The Windows firewall doesn't stop malware on your pc from dialling out. Either of these firewalls are free for personal use and stop unwanted incoming and outgoing traffic. You choose which programs are allowed to access the internet. They are both fairly easy to use and updated regularly.
http://www.zonelabs.com Zone Alarm firewall
http://www.kerio.com Kerio firewall
Shields Up security site, check your firewall works
https://www.grc.com
http://www.mozilla.com/ Firefox web browser
http://www.opera.com/ Opera web browser
Instal both these web browsers, turn off javascript on one and use that browser for vising unknown websites. They don't support Active X so thats one less way for problems to get on your pc.
Winpatrol is a handy utility that tracks cookies on your pc and stops unwanted changes to your startup folder.
http://www.winpatrol.com
Spywareblaster works with Internet Explorer and Firefox. It won't uninstall spyware that is already on your pc, but it will prevent hijacks and other malicious changes to your browser.
http://www.javacoolsoftware.com
http://www.siteadvisor.com -Site Advisor warns about malware on sites you are about to visit.
http://www.avast.com - Avast Antivirus
http://www.trendmicro.com/vinfo/ - Trend Micro online virus guide
http://housecall.trendmicro.com/ - Trend Micro PCillin online scan
http://www.ewido.net - Ewido anti trojan
Ad-Aware SE Personal anti adware
http://www.lavasoft.com
Spybot Search and Destroy anti spyware
http://www.safer-networking.org
http://spywarewarrior.com/rogue_anti-spyware.htm - The Spyware Warrior site identifies fake anti spyware programs.
http://www.bitdefender.com -BitDefender can be used as a second, on demand anti virus.
http://www.ccleaner.com Cr*pcleaner Registry cleaner and privacy tool
Microsoft have two free online scanners that look for malware.
http://safety.live.com/site/en-US/default.htm
http://www.microsoft.com/security/malwareremove/default.mspx
You can also download Microsofts free Windows Defender from the official Microsoft site, and the Malicious Software Removal Tool.
Remember to update and run all of you anti malware programs at least once a week.