Microsoft has admitted - again - that the upcoming Windows XP Service Pack 2 (SP2) will break applications but has agreed to provide free worldwide technical support. The major changes in SP2, officially due in the third quarter but which could be available as soon as next month, are expected to cause support headaches. Analysts, users, PC makers and Microsoft are all expecting a spike in help desk calls as a result.

The service pack, which will be downloaded automatically onto many PCs through Microsoft's Windows update service, could break current applications, disrupt networking setups and prompt non-technical users to make PC configuration decisions that may be beyond their grasp. However, Matt Pilla, a Microsoft senior product manager, said: "It represents one of our most broadly-tested products to date".

Microsoft has admitted there is a trade-off with SP2, focusing on security at the expense of compatibility, and has warned that the XP update could break existing applications.

Because of the extensive changes that Service Pack 2 includes, the software giant is urging developers and IT professionals to test the update thoroughly. The update is in fact more of an upgrade than a service pack. It contains bug fixes and updates, but also offers new features and makes significant changes to the Windows software in four main areas: network protection, memory protection, e-mail security and browsing security.

All users will notice Service Pack 2's changes to Windows. For example, Windows Security Center will alert a user if his system isn't sufficiently secured. Also, a security wizard pops up after installing the update, asking a user to make decisions about settings such as automatic updating. Another new feature is a setup tool for wireless networks.

Consequently, all users of Windows XP may have reasons to grab the phone and call support. "The first time Windows Security Center pops up and says your computer is not secure, that can lead to a panic attack," said Joe Wilcox, a senior analyst at Jupiter Research. "Or what if a warning you've never seen before appears when downloading a Web page?"

Microsoft recognises that there will be an increase in support demand. "I definitely think we will see ramped-up support requests," Microsoft spokesman Tom Pilla said. "What we have here is a security-focused service pack release that is going to have some significant impact on people's systems." Meanwhile, Microsoft is planning its support for the update but has decided to offer no-charge, worldwide telephone support for the service pack.

Pilla said Microsoft was returning to its policy to provide free support for service packs after leaving support for Windows XP SP1 to the PC makers. It doesn't yet know how it will deliver support though. It could establish a dedicated, toll-free support line, Wilcox suggested.

Support direct from Microsoft is more valuable than help provided by PC vendors because it is more in-depth, said Victor Go, vice president of technology at Landmark Theatre, which has about 600 PCs running Windows. "We don't call unless it is something extremely technical that would require something beyond the first-line helpdesk. We used to try the support that goes with the PC, but whether it is HP or IBM, we just never got the response we needed," Go said. Nevertheless, HP and Dell are also gearing up for the release of SP2 and will support their customers, spokespeople for those PC makers said.

Users are nervous. Microsoft will have to treat SP2 like a new operating system release, said Rob Helm, a director of research at industry research company Directions on Microsoft. "It is that level of change," he said. Go said: "Businesses like us don't run the latest version of an operating system. We did not roll out XP until almost a year after it came out. It is kind of scary that in order to get the required updates, we also get all these enhancements, which is usually a separate project."

The changes combined with the automatic update are especially worrying to Thomas Smith, manager of desktop engineering at a large Houston-based company. During testing of the service pack, he found that the 5,000 or so Windows XP desktops he manages will no longer be able to connect to the home office with the update installed.

"It will break our enterprise to a point," he said. The PCs use the Internet and Cisco VPN software to connect. What's more, because SP2 switches on the Windows Firewall by default, Smith's remote management tool, supplied by Altiris, can't connect to the PCs to correct the problem, he said. "We will have lost contact with our end users through Altiris because the ports will be locked," Smith said. "With the firewall being flipped on, our ability to fix any problems without having end users call the help desk is gone, and helpdesk is money."

With security being one of Microsoft's top priorities, the company has set aside marketing dollars and is planning campaigns online and with PC makers to get users to update their systems, Rich Kaplan, a corporate vice president in Microsoft's security business & technology unit, said. "The benefit of getting people to SP2 outweighs any increased support burden," Kaplan said. He emphasised the urgency for IT professionals and developers to download and test SP2 and report any issues to Microsoft. Directions on Microsoft's Helm agreed. "This is the right thing to do," he said. "Users continue to be whacked by worms."

A first beta of the Windows XP update was released in December, followed by RC1 in March. A second release candidate, probably the final test version, was made available earlier this month. Hundreds of thousands of developers and IT professionals have already tried out the software.