U can find lot's of free security systems just by using google. Myself I use AntiVir Personal edition (I think it'salso called AntiVir /XP) and spybot s&d, ad-aware and spywareblaster for anti-spyware. I have a firewall in my router so I don't need a program but I used to use zone alarm pro.
All of the security programs I mentioned are free.