There are numerous Antivirus softwares in the market. I have seen people buying the expensive one thinking it will do the best job, but its untrue. I came across antivirus from Norton, McAfee are the common ones used, but same time they take up so much resource that it slows down your pc. I have been managing networks of 120pcs daily. From last 2 years I have been using Free Antivirus on 20-40pcs in my network and the result is superb, and its free of cost.

Try these free antivirus and forget about paying some to Norton or McAfee.

Avast! 4 Home Edition

Avira AntiVir Personal

AVG Anti-Virus Free Edition

Trend Micro's FREE online virus scanner

Comodo Firewall and AntiVirus

Ad-Aware (free Anti-Spyware)

SUPERAntiSpyware (free Anti-Spyware)

my personal recommendation is AVAST or AVIRA.