Fire up a desktop computer at many small or medium-size businesses, and there is a decent chance it is running on a 12-year-old operating system: Windows XP.

But starting next April, Microsoft will no longer offer security updates for that venerable software to protect it from newly emerging viruses, spyware and other malware.

"When XP goes out of support, it'll be a lot like driving a car that you can't buy parts for anymore," said Jay Paulus, Microsoft's director of Windows small-business marketing.