Beginning Saturday, Microsoft will send warnings it will stop issuing updates for the XP system, which was first released in 2001 and today still operates 30 percent of all desktop personal computers.
"I've been in security responses for 15 years and I've never seen anything like this. It is my hope everything will be OK but I am truly concerned," said Christopher Budd, a former Microsoft employee now a global-threat communications manager for anti-virus software maker Trend Micro.
Without constant security updates, hackers can find ways to infiltrate XP-run computers, he said.
Microsoft patched about 100 vulnerabilities to the XP system in 2013, the San Francisco Chronicle reported Sunday.
The XP system was a product of 1990s-era technology. Long before the development of cloud computing, Internet media and mobile devices, said Tom Murphy, Microsoft's Windows communications manager.