Many people using pirated copies of Windows XP will get no help from Microsoft to make their PC safer.
Most legal users will be able to use the patch
The software giant has decided that a forthcoming update to XP will not work with the most widely pirated versions of its operating system.
The upgrade, called Service Pack 2, closes security loopholes in XP and adds features that make it easier to keep machines safer from viruses.
The software update is due to be released during the summer.
SP2 is the long awaited upgrade for Windows XP that Microsoft hopes will make the software much more resilient to many of the ways that malicious hackers and virus writers have exploited it before now.
Also included are features that make it easier for users to manage their anti-virus software and firewall. It also forces users to make explicit choices about how secure they want their PC to be.
Other features include a blocker for adverts that pop-up when people browse the web and background utilities that warn when spyware is trying to install itself on their machine.
Once installed SP2 also changes the way that future updates are installed.
Instead of downloading the whole chunk of XP being updated, SP2 instead only downloads the parts that have changed.
This change should reduce future patch download times by up to 80%.
The arrival of SP2 also has implications for those running websites and Microsoft has issued advice to help webmasters cope with the changes.
Paul Randle, Microsoft's UK manager of all things XP, said the final SP2 package would be about 80MB in size when released.
"It is not a normal service pack," he told BBC News Online. "We are breaking our own rules that said we would not put new code into service packs."
Piracy is big business in many nations
Microsoft was working hard to ensure that users could get hold of the software as many ways as possible, he said.
Net service providers plus software and hardware partners of Microsoft are expected to make copies available to customers and subscribers.
Users will also be able to register on a Microsoft website to get a CD containing the patch sent to them.
Mr Randle said during installation SP2 will check the product ID number for the copy of XP in use on a PC and will not let itself be installed if that software is a version that has been widely pirated.
Microsoft has worked out the 20 most pirated product IDs and SP2 will not install and run on any copy of XP bearing one of those numbers.
"The situation at the moment is that we will block those," he said.
It is unclear what effect this strategy will have in countries where much of software used is illegal. For instance, the anti-piracy Business Software Alliance estimates that 92% of software in China is pirated.
Mr Randle said Microsoft was keeping its SP2 strategy under constant review.
"Whether it will change between now and launch I do not know," he said.
Service Pack 1 for Windows XP worked with almost all legitimate and pirated versions of the software.
Only those copies of XP that used the two most widely pirated product IDs were barred from getting the upgrade.