A common misconception I run into is the confusion between anti-virus software and anti-malware/anti-spyware programs. Although there is some overlap in detection between anti-virus and anti-malware programs available today, a dedicated anti-virus software is still a must have on your computer.
I am frequently asked in the forums to recommend a free anti-virus software. The two "free for personal use" programs that I recommend are Avast Home Edition and AntiVir Personnel Edition.
Earlier this week HowToGeek published a guide for installing and setting up AntiVir. The second installment in the series on Avast was published today. If you need an anti-virus software, one of these may be the solution: