The widespread failure of Skype's Internet telephony service last week happened when millions of Windows users tried to log in to the system at the same time, after downloading a software update from Microsoft and rebooting their machines, Skype said Monday.

Users encountered problems logging on to Skype's VOIP (voice over Internet Protocol) service early Thursday morning, leaving them unable to connect until Saturday.

Skype said that the load placed on its system as computers rebooted after receiving a routine set of patches from Microsoft's Windows Update service revealed a previously unknown bug in the Skype software.

Although the Skype network has a built-in self-healing function, the bug within the network resource allocation algorithm prevented the function from working as designed, according to the company.

Skype has identified the bug, which was a design error and not caused by hackers, it said. The company has already introduced "a number of improvements" to its software to help resolve the problem.

However, Skype provided no details on why Microsoft's monthly Windows upgrades have not caused problems in the past, nor whether future upgrades could trigger a similar network outage.

Knowing exactly what happed to the Skype network is difficult because the company uses proprietary technology and says little about it in public.

While analysts agree that Skype has a relatively good service track record, the company needs to be careful that outages of this type don't repeat themselves.

"They'll get away with this one or maybe even twice but if it becomes a succession of events, then people will say it's not for me," said Mark Main, senior analyst with Ovum.

The outage, Main added, is not beneficial to the goals of Skype's parent company, eBay, which has said that it wants the VOIP operator to make more money from existing customers rather than enlarge its customer base.

Ian Fogg, senior analyst with Jupiter Research, said Skype needs to act quickly to regain customer trust. "Peer-to-peer VOIP companies like Skype need to take another look at their engineering resources and slightly take the foot off the pedal and make sure that what they do is reliable," he said.