A two-day outage that left millions of Skype users unable to use the popular Internet phone service was caused by an abnormally high number of restarts after people had downloaded a Windows security update, the company said Monday.

In an update to users on Skype's Heartbeat blog, employee Villu Arak said the disruption was not because of hackers or any other malicious activity.

Instead, he said that 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," Arak wrote.

Microsoft Corp. released its monthly patches last Tuesday, and many computers are set to automatically download and install them. Installation requires a computer restart.

"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," Arak wrote.