Viruses have been spreading like wildfire lately. A lot of people have no anti virus software installed because it either costs too much, it slows down their computer too much, or it has simply expired. I recommend that you give Security Essentials from Microsoft a look.

This solution is free, never expires, doesn't slow down your computer, and it has detected and removed malware that was not detected by Norton or Mcafee! This is simply put, the most effective anti virus solution for Windows that I've come across yet.