You can try AVG antivirus -
http://free.avg.com/
It includes anti-virus and anti-spyware capabilities. Should be enough for basic protection.
If you do not have firewall installed, maybe a good idea to install a free one. I believe either Zone Alarm and Comodo are very good.
Zone Alarm -
http://www.zonealarm.com/security/en...e-firewall.htm
Comodo firewall -
http://personalfirewall.comodo.com/
Please do note that Comodo product includes firewall and antivirus. I installed the firewall part only as per comments, the antivirus is not that effective against viruses.
Hope this helps.