Microsoft has issued a mandatory fix for vulnerabilities in Windows Live Essentials running on Windows XP.
Piero Sierra, group programme manager for Windows Live, said in a blog post that the update contains important security fixes, performance improvements and "minor changes".
The update was released as an optional download in May 2010, but has since become mandatory, suggesting that unpatched versions of Windows XP may now be insecure.
Microsoft originally described the update as a "minor release with a small set of targeted bug fixes".
"Even though most users will not run into the issues we addressed, we felt they were important to fix," said Arthur de Haan, director of Windows Live test and system engineering at Microsoft, at the time.
It is likely that users of untouched versions of Live Messenger will lose the ability to hold one-way video conversations, something that caused controversy when announced in May.
De Haan explained in May that the changes were necessary as they cleared a path to higher quality two-way calls.
"The next version of Windows Live Messenger features two-way calling with high-quality audio and high-definition video," he said.
"This enhancement required significant changes in the audio and video stack used across all of our Windows Live services. As part of this upgrade, we are retiring one-way webcam functionality."
The update is rolling out to users this week.
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A survey of local authorities has found that they face challenges in the areas of data, compliance and mobility.
More than 800,000 home users could be affected