Microsoft may borrow the technology used by malware authors to distribute software patches and updates. 

Friendly worms could seen be used to self-replicate and spread to new computers, distributing updates to users who otherwise might not download them.
An article in New Scientist details how the new worms, under development at Microsoft Research in the UK, woudl seek out vulnerable machines and patch them before malicious code has a chance to attack them.
This could lead to "worm races" where malware and "friendly" worms attempt to reach vulnerable PCs first.
However, researchers hope the friendly worms will be more intelligent that their malicious counterparts as they would be capable of learning from experience.