Software giant Microsoft has fully rolled out its crucial security update to computers running its Windows XP.
Microsoft said it was crucial PC users update
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.
He said it meant 100 million would be automatically updating systems.
"We have improved web browsing experience in Internet Explorer," he said: "It now has a pop-up blocker by default.
"Also some sophisticated work in Internet Explorer has been done so it is impossible to write windows that hide behind others."
Pop-up ads blocked
Revamped firewall on by default
Outlook Express, Internet Explorer and Windows Messenger warn about attachments
Origins of downloaded files logged
Web graphics in e-mail no longer loaded by default
Some spyware blocked
Users regularly reminded about Windows Updates
Security Centre brings together information about anti-virus, updates and firewall
Protection against buffer over-runs
Windows Messenger Service turned off by default
This is often a ploy used to deposit spyware onto and malicious code onto people's machines without them realising.
He stressed it was crucial that XP users, business and individual home users, switched on their auto-updated so that machines could download SP2.
At 80Mb, it is a large file to get for many; on broadband speeds, it could take over an hour. Dial-up customers would need longer.
But, said the spokesperson, added features to the software meant that users could start the download, and "forget about it".
"Service Pack 2 will be downloaded in the background; it is aware of how much bandwidth you have so if you are in the middle of downloading e-mail it will let you do that," he said.
"We have also enabled a check point restart which means if you break the connection in the download, it remembers how far you got."
The SP2 update for those using the Home Edition of XP was made available last week as a "trial", said the spokesperson, but Microsoft did not make it available automatically to all until Wednesday.
The corporate version of XP was delayed because many large networks wanted more time to prepare for the disruption to other programs the updates might have caused.
SP2 can also be ordered on a CD or found on cover CDs with some consumer technology magazines.