A tenacious computer worm which has wriggled its way onto machines worldwide is set to evolve on April Fool's Day, becoming harder to exterminate but not expected to wreak havoc.

A task force assembled by Microsoft has been working to stamp out the worm, referred to as Conficker or DownAdUP, and the US software colossus has placed a bounty of $250,000 on the heads of those responsible for the threat.

The worm is programmed to modify itself on Wednesday to become harder to stop, according to Trend Micro threat researcher Paul Ferguson, who is part of the Conficker task force.

"There is no evidence of it going into attack mode or dropping any particular payload on April 1st," Ferguson said in an interview.

"What people controlling the botnet are doing is building in survivability because of efforts by the good guys to lessen the harm of this thing," he said.

The worm, a self-replicating program, takes advantage of networks or computers that haven't kept up to date with security patches for Windows RPC Server Service.

It can infect machines from the Internet or by hiding on USB memory sticks carrying data from one computer to another. Once in a computer it digs deep, setting up defenses that make it hard to extract.