Internet phone service Skype restored service this weekend after a routine software upgrade left millions of users unhooked since last week.

The service, founded by the Scandinavian entrepreneurs behind file-sharing service Kazaa and now owned by online auction company eBay, restored service on Saturday and explained the outage on Monday on its Web site.

"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 company said on its blog.

The high number of restarts revealed a bug in its network resource software, preventing a "self-healing" function of its software from working, the company said.

Skype said no malicious activities caused the problem.