Years ago the morris worm had thousands of PCs infected and there were even more paraniod PC users disconnecting their PCs from phonelines.

Here is a example of how worms function.

Like almost any evil code worms have active-X features which load the program and then mail the worm.

Code is attached to the main subprogram: Private Sub Form1_load then the attached code tells the computer to open, close, edit, & delete files.

Since worms mail themselves that means that even if the worm isn't deadly to PCs it can still mail itself and move in swarms intil it becomes a DoS attack on e-mail accounts.

You can prevent this by takeing a very close look at the programs you own and check for viruses and worms with anti-virus softwares.