Last week, it "soft launched" Service Pack 2 (SP2) security update, making it available for some home users but not all, through its auto-update service.
But from Wednesday, PCs running XP's Home and Professional software will be able to get the auto-update.
The update is supposed to provide more protection against malicious viruses and make it easier to manage security.
The SP2 update, Microsoft said, should make it harder for virus writers and malicious hackers to compromise a computer's operating system.
It makes some changes to the way XP works, but also lets PC users monitor and adjust security setting from one place.
A Microsoft spokesperson told BBC News Online that it was a "major and significant security update for Windows" and that it was essential PC users running XP updated their software.