By Alfred Hermida
BBC News Online technology editor
File-sharing activists are using Microsoft's key update for Windows XP to highlight the benefits of peer-to-peer technology.
Microsoft is slowly rolling out the update to Windows users
A lobby group called Downhill Battle has set up a link using file-sharing software to distribute the SP2 update.
Microsoft told BBC News Online it was investigating the site.
The SP2 update has already been released to PC manufacturers and is expected to be widely available to home users for free in late August.
Microsoft hopes to have the Service Pack 2 security update installed on more than 100 million machines in the next two months, but it is limiting daily downloads of the software in order to prevent an overload on its servers.
The file-sharing lobby group, Downhill Battle, has taken matters into its own hands. It has made a copy of SP2 available using BitTorrent file-sharing technology.
Using BitTorrent software, users get pieces of the file from the original server, and from anyone else who is downloading.
CHANGES DUE IN SP2
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 Center brings together information about anti-virus, updates and firewall
Protection against buffer over-runs
Windows Messenger Service turned off by default
It means that other people looking for the same file can start downloading more quickly.
"Even Microsoft, which has incredible server resources at their disposal, is limiting downloads of their SP2 release, but file-sharing technology can let everyone download it right away," said the group on its site.
"This project shows how file-sharing technology gives people without budgets or huge server space the power to solve problems themselves, without waiting for the government or some corporation to do it for them."
In a statement a Microsoft spokesperson said: "The Microsoft Download Center site is the only authorized web source for downloading a licensed copy of Windows XP Service Pack 2."
They added: "To report a website offering unlicensed copies of Windows XP SP2 for download, please send an e-mail to email@example.com"
The version of SP2 offered by Downhill Battle is intended for computer professionals and developers downloading and installing multiple computers on a network, rather than for home users.
Microsoft is advising customers not to download this version, which weighs in at 266MB, if they are updating just one computer.
It says that a smaller, more appropriate download will be available soon on Windows Update service.
Microsoft has advised users to turn on Windows auto-update to make sure they get the right version of SP2.
The update was originally supposed to appear in 2003 but successive technical hitches have delayed its arrival.
It is designed to close many of the loopholes that Windows viruses have exploited over the last few years.
It installs a security centre that lets people see and manage the measures installed on their machine to block viruses and hack attacks.
The update also blocks pop-up ads and lets users know when spyware is trying to install itself on their machine.