After that date, Microsoft will no longer distribute official security updates or bug fixes for the ancient operating system -- though it will offer them to enterprises that pay for high-priced support contracts.

PCs still running XP won't just suddenly stop working, of course, but they will be vulnerable to attacks exploiting weaknesses in the operating system that are discovered after that date.

Some observers believe that leaving PCs running the still widely used operating system could lead to trouble. Michael Cherry, an analyst at Directions on Microsoft, conjured up this scenario: What happens if "a security problem with XP suddenly causes massive problems on the Internet, such as a massive [denial-of-service] problem?" That could become a worldwide tech crisis, and "in this scenario," he said, "I believe Microsoft would have to do the right thing and issue a fix."