Microsoft yesterday claimed it had fixed the problem that had kept some users from logging on to its online services. And Microsoft said it could state with absolute certainty that the launch of Windows 2008 had nothing to do with the problems.

The company has, however, yet to explain the outage, which left users unable to access everything from Windows Live Mail - formerly Hotmail - and Xbox Live to Skydrive Live and Live Messenger.

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Coincidentally, hours before the outage began, Bob Muglia, the head of Microsoft's server group, bragged that "a good part of the Windows Live servers are running Windows Server 2008". The latter, Microsoft's newest server operating system, launched yesterday.

Yesterday, however, Microsoft said the problem had been resolved and all services were available.

"Earlier today, an issue began that has caused some consumers worldwide to experience difficulty logging in to their Windows Live ID accounts," said a statement attributed to Samantha McManus, product manager for Windows Live.

"This issue has since been resolved and normal operations have been restored to all customers."

As of 6.20pm in London, however, PC Advisor readers were still complaining of not being able to log in.

By mentioning Windows Live, McManus hinted that the problem was across most if not all of Microsoft's online services that require users to log in.

Windows Live ID - once known as Passport - is Microsoft's single sign-on service. Most of the sites and services that rely on Live ID are owned and operated by Microsoft, although a few non-Microsoft sites, such as Expedia, also use it. Expedia representatives were unavailable for comment yesterday.

"The issue purely impacted the log-in process for customers and largely did not impact customers who were already logged in," added McManus.

"Microsoft worked aggressively to resolve this unique problem as quickly as possible. We apologise for any inconvenience this issue has caused consumers."

According to messages left by people in support forums and as comments on numerous blogs, Windows Live ID was knocked out for users living in the UK, Finland, Honduras, Canada, Australia, the US, and places in between and beyond.

This is not the first time that multiple Microsoft services have been crippled simultaneously. A year ago next month, Live Messenger, Live Mail and other sites and services were inaccessible to users for part of a day. Last April, Live Mail and Messenger also went dark for several hours. In neither incident did Microsoft reveal the underlying cause.

In a story that went live very early Tuesday on CNET's, Muglia said that Windows Server 2008 did not have a "live" element of its own, then continued on to note that the OS powered an unspecified number of Microsoft's sites and online services.

"The biggest 'live' component is the fact that Windows Live runs on Windows Server," he said. " is running entirely on Windows Server 2008 right now."

CEO Steve Ballmer is scheduled to officially launch Windows Server 2008 today.

Microsoft dismissed the idea that the Live ID blackout was related to the new server software, though it neither confirmed or denied that the log-in service was hosted on Server 2008.

"We can say with certainty that Windows Server did not contribute to this incident," a company spokesman said in an email today.