Parts of the Microsoft MSN network were unavailable on Sunday in a serious outage that also closed down MSNBC.com and Newsweek.com, which have hosting agreements with the Redmond giant.
Users were unable to access chat rooms or the popular Game Zone, and some Hotmail users reported difficulties with the Passport log-in system.
Internet Explorer 6 users were particularly affected and could not search the web using the default setting. The main MSN site was not affected, nor was the Microsoft website or its .Net message service.
Experts warned that not being able to log out may have created potential security weaknesses. The problems were not resolved until late in the afternoon.
Although Microsoft has not yet explained why the sites went down, technical experts said that it appears to have been a problem with one of its primary backbone routers.
It is the second major outage for MSN in a few weeks. Last month a server glitch locked Hotmail users out of their accounts, and in January some users were blocked from accessing Microsoft's games site.
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