After a few notable delays, Microsoft has finally shipped the latest beta version of Windows XP Service Pack 2, which the company has touted as a major milestone in its efforts to develop more secure software.

The second "release candidate" of the service pack, which more resembles an upgrade to the two-year-old operating system, was made available late Monday in the US to beta testers in Microsoft’s Technical Preview program. Microsoft intends to allow a wider audience to download Release Candidate 2 today.

As for the final release of the service pack, Microsoft would only say it is expected this summer. The final release of SP2 was originally planned for the first half of this year.

Microsoft released the first beta of Windows XP SP2 in December, followed by Release Candidate 1 in March. Release Candidate 2 was originally planned for last month, but Microsoft was said to have delayed the release to fix some bugs and correct some compatibility issues.

Microsoft has warned corporate users for months that SP2 will break some applications. The company has been imploring users to thoroughly test applications against the service pack.

The application issue is most prominent in the Windows Firewall, which is turned on by default and will disrupt communication for certain applications, such as remote administration and patch management tools, performance monitors and other tools that communicate over file-and-print sharing channels, hard-drive shares that operate over specific ports, and peer-to-peer and file-sharing programs. The firewall can be configured to permit those communications, but users will trade security for those conveniences.

Also, new security restrictions placed on Remote Procedure Call (RPC) and the Distributed Component Object Model (DCOM) services, which are often exploited by worms and viruses, could choke existing applications, as may new memory protection features that will stifle code generated by just-in-time compilers. Those issues and others, such as recurring exploits of Windows, have generated a lot of hubbub over SP2.

"We are not lying in wait for this service pack," says Roy Haschenburger, president of Alternative Computers, an IT contractor to government organisations. "It’s a non-issue right now. We have tested it and haven’t seen any real positive or negative impact. I think people are concerned with day-to-day issues and such things as worms rather than worrying about Microsoft coming up with something that might improve the OS and make it more robust and stable." Microsoft has touted those goals, along with security for the service pack.

In addition to the Windows Firewall, key highlights include safer Web browsing features, including enhancements to Internet Explorer to block pop-ups and unintended downloads; memory protection to reduce buffer-overflow vulnerabilities; and safer e-mail and instant messaging through better protection against malicious attachments and Instant Messenger file transfers. Also, the Messenger Service, a network administration tool that has been used by spammers to send pop-up ads to users, will be turned off by default.

For corporate users, several of the features of the service pack can be centrally administered through Active Directory Group Policy, including the firewall and pop-up blocking.

Microsoft says hundreds of thousands of developers and business customers have been testing beta and release candidate versions of Windows XP SP2 through Microsoft's technical preview, technical beta and MSDN programs.