This is why one uses Aniti-Virus programs and is careful on what you open up attachment wise in emails. Also when you download something make sure you scan it with your anti-virus.
This is screen shots from my son's girlfriends computer. A scan showed 35 Virus which is bad enough but this one got her a one way ticket to reformatting (which I'll be doing this afternoon on her computer):
This is not good! She DID have a anti-virus but it was shut down by one of the many trojans/worms/virus that she had.