The Sasser worm is spreading quickly. Unlike worms and viruses which spread through e-mail attachments and other avenues, this worm hits any PC connected to an internet service provider. People with firewalls will most likely not be affected.
So far, the worm has been given a "low" rating since all it does is shuts down computers and restarts them, repeating this process a few times. Security experts think the worm was created by a teen who wants an innocent prank to show skills off with.
Graham Cluley of anti-virus firm Sophos says "The Sasser worm spreads in a similar way to last year's serious Blaster outbreak, in so much as it travels via the Internet exploiting security holes in Microsoft's software and does not use email." Source