The last few major virus scares have happened through e-mail attachments, written in a language called Visual Basic. They just need to know the proper commands, and have the required motivation. Back in the late 1980s, I taught Computer Science at a private K-12 school in Fort Worth, Texas. His related term project was stored on the same hard drive as the term project of independent study students in another class. Such is the mindset of many virus authors.
Hoaxes play on our inborn fears, and also on our desire to help others avoid danger.
If it passes all these tests, then it's best to take the warning seriously - but most such warnings fail these tests. Unfortunately, yes there are. Computer viruses, though not as common as many people think, are very real. Yet this particularly nasty virus shook up computer users worldwide within a matter of days, thanks to its friendly disguise and its means of self-propagation. Any reasonably good virus sweeper can take care of this one.