A helpful worm is variant on a computer worm which delivers its payload by doing "helpful" actions instead of malicious actions. Welchia was a major example of a helpful worm - utilizing the same exploit that caused the Blaster worm. Infected computers automatically began downloading Microsoft security updates for Windows without the users' consent and automatically rebooted the computers. One of these updates was, incidentally, the patch that fixed the exploit.
Even though they do not do anything "malicious", most helpful worms do not log events, and they automatically reboot the computer as part of the installation process without the user's consent. They also can put strain on the network as they spread and download updates.