Those who use Service Pack 1, which is now 4 years old, must upgrade or lose support as of Tuesday, Microsoft says.
The move to drop support for SP1 is in line with Microsoft's stated strategy for support. According to its guidelines, Microsoft guarantees to provide "mainstream support" for a full product for five years, but will only guarantee to support a service pack for 12 months after the launch of the next version of that pack.
SP1 shipped in September 2002. SP2 was released in September 2004.
There's little reason for anyone to still be running SP1; SP2 contained a range of improvements to XP's security. People can check which version they are running by right-clicking on the My Computer desktop icon and then selecting properties.
The ending of support for SP1 will help Microsoft to clear the decks for the arrival of Vista, which is expected to be released to business customers next month and to consumers in January.
In July, Microsoft ended support for Windows 98 and ME.
Colin Barker of ZDNet UK reported from London.