The attack on Microsoft's Windows Update service at the weekend failed to cause significant problems due to an elementary mistake by the virus writer.

The Blaster worm, which has been in circulation for a week, attempts to use infected PCs to launch a denial of service attack against the Microsoft website where customers download software patches.

The worm targeted windowsupdate.com for overload on the sixteenth of every month.

But this is not the main address for the service, which is actually at windowsupdate.microsoft.com.

So all Microsoft had to do was remove the redirect that previously would have sent users to the main download site, to avoid the brunt of the attack.

"That was one of a number of steps we took," said Microsoft UK's chief security officer, Stuart Okin.

"It was a busy weekend nevertheless, with our support engineers working overtime to deal with the high levels of support calls.

"We'll carry on monitoring the situation regarding further attacks and will be prepared."

However, the main Microsoft website did suffer separate attacks, unrelated to the worm, on Thursday and Saturday. The main site was down for two hours on Thursday and for just under an hour on Saturday.