Earlier Tuesday, Windows Update, Microsoft's primary update service, began delivering the first patch to Windows 7 since the company struggled to launch the public beta last Friday. The update fixes a flaw that shaves several seconds of audio from any MP3 file that's edited, including files modified automatically as users connect to the Internet.

"Without action on your part, all MP3 files that have large headers in your Windows Media Player and Windows Media Center libraries are likely to lose some audio," Microsoft said in the support document it published Saturday, several days after it first posted the fix to its MSND and TechNet subscription services.

Before Tuesday, users who wanted to apply the fix had to find it, download it manually and install it themselves.

Microsoft also recommended that users back up all MP3 files before doing an upgrade to Windows 7 from Windows Vista, and that they set all of them to "read-only" status by right-clicking each file in Windows Explorer and then clicking the General tab and selecting the "Read-only" box. Failing that, users should disable metadata automatic updates in Windows Media Player, Microsoft said.

At the same time, it quashed the MP3 bug, however, Microsoft ignored a vulnerability in the Server Message Block (SMB) protocol that affects every version of Windows, including Windows 7.

Microsoft explained why the flaw went unfixed. "We provide security updates for beta versions of Windows through Windows Update for Critical issues only," said Christopher Budd, a spokesman for the Microsoft Security Response Center (MSRC), in a post to the group's blog Tuesday. "So the vulnerability will be addressed in the next public release for Windows 7."

Of the three bugs patched by the MS09-001 security update today, just one is pertinent to Windows 7, Budd added. That vulnerability, designated as CVE-2008-4114, is a denial-of-service bug rated "moderate," the second step in Microsoft's four-level scoring system.

The remaining two vulnerabilities -- both labeled "critical" by Microsoft -- affect Windows 2000, XP and Server 2003; one of them also affects Windows Vista and Server 2008.

Windows 7 beta, which was released Saturday, will be available for download through at least Jan. 24.