The outage comes barely a week after the software giant experienced similar sign-in problems with the e-mail service and the instant-messaging service. Both use Microsoft Passport, a universal password and authentication software that is incorporated into MSN's family of Web sites.
Larry Grothaus, MSN's lead product manager, said in a statement that the current outage, first discovered by Microsoft at 9:30 a.m. PST, was not connected with the previous sign-in difficulties. He added that the slowdown was not related to Passport but did affect some Passport services.
"Customers may continue to experience sporadic issues in the near term, but we anticipate having service fully restored to all customers very soon," Grothaus said in the e-mailed statement. "We sincerely apologize for any inconvenience or disruption this may cause our customers."
Grothaus said Microsoft was working on fixing the "issue" that caused the slowdown but did not give any further details of the cause.
After last week's outage, a representative of Keynote, which monitors Internet performance, said the sign-in problems could have stemmed from Passport. The representative said the outage was limited only to MSN sites.
A Keynote representative said the firm did not notice any significant slowdowns on MSN.
Microsoft has hailed Passport as the centerpiece of its Web services strategy. The software stores personal information, such as birth date and sometimes credit card information, to enable people to log in quickly to any Passport-supporting Web site. Having a central sign-in service lets people manage their personal accounts, purchase goods and services or access popular features such as e-mail and instant messaging.
Third-party Web sites such as those of eBay, USA Today and Starbucks also support Passport.
CNET News.com's Rob Lemos contributed to this report
