Microsoft's Web site was made inaccessible for an hour and 40 minutes Friday afternoon when a denial-of-service attack overwhelmed the site with traffic, making it impossible for legitimate page requests to get through.

The outage, which began about 1:21 p.m. Pacific time, was the result of a conventional denial-of-service attack and not a software vulnerability being exploited, a Microsoft spokesman says.

That distinction is important because the software company issued a bulletin July 14 warning customers of a critical vulnerability in its Windows NT, Windows 2000, Windows XP, and Windows Server 2003 operating systems. Following that notice, the Department of Homeland Security issued its own advisory warning consumers and businesses to patch their computer systems.

The denial-of-service attack affected Microsoft's home page, www.microsoft.com, and the many other URLs associated with it, including Microsoft's tech-support page, www.support.microsoft.com.

Microsoft personnel are working in conjunction with law-enforcement officials to trace the attack. The spokesman points out that a hacker's conference opened Friday in Las Vegas. But he said Microsoft had no evidence to link the denial-of-service attack to the conference or to warnings three weeks ago of a broad, coordinated attack against Internet sites.