On the Skype blog, Villu Arak writes: "On Thursday, 16th August 2007, the Skype peer-to-peer network became unstable and suffered a critical disruption. The disruption was triggered by a massive restart of our users' computers across the globe within a very short timeframe as they re-booted after receiving a routine set of patches through Windows Update.

"The high number of restarts affected Skype's network resources. This caused a flood of log-in requests, which, combined with the lack of peer-to-peer network resources, prompted a chain reaction that had a critical impact.

"Normally Skype's peer-to-peer network has an inbuilt ability to self-heal, however, this event revealed a previously unseen software bug within the network resource allocation algorithm which prevented the self-healing function from working quickly. Regrettably, as a result of this disruption, Skype was unavailable to the majority of its users for approximately two days."

Arak assures users that no malicious activities were attributed to the issue, with no security risk, and that Skype has upgraded its software to avoid a similar incident in future. 

What Skype has not revealed, however, is why this particular batch of updates caused the disruption, since Microsoft releases new security updates on the second Tuesday of each month.