Microsoft has released updates to improve Windows Vista's speed and reliability, for the second time in as many months.

The four separate fixes address several operating system performance and stability problems, as well as deal with a dozen Universal Serial Bus issues, improve Windows Media Player and patch Media Centre.

Although Microsoft has put Windows Vista Service Pack 1 (SP1) in the hands of some testers, it said it will continue to update the original Vista - dubbed "RTM" for "release to manufacturing" - even as it puts SP1 through its paces.

Seven weeks ago, it issued a pair of updates that tackled numerous problems, offering them as optional items through Windows Update.

"[Tuesday's] updates are a collection of fixes that we have made to address a small set of reliability, compatibility, stability, security and performance issues," a Microsoft spokeswoman said in an email. "[They] will provide incremental improvements to the most common issues - but in general, these improvements or fixes are going to be very narrow in scope."

The widest ranging of yesterday's quartet was a 5.4MB update that Microsoft said extends laptop battery life, improves the stability of wireless connections and deals with compatibility problems with some anti-virus software.

Interestingly, the update also promised to shorten Vista start-up and resume-from-sleep times, problems that Microsoft had supposedly fixed with the August patches. Vista users have complained about Vista's slow start-up, shut-down and return from power-saving modes since at least April.

A second update is a cumulative roll-up of 12 fixes to Vista's USB components. In Microsoft's terminology, a "roll-up" is a collection of patches, similar to a service pack, but not tested as extensively. A similar cumulative update for Media Centre is also available from the Microsoft download site; it deals with several specific problems, including some involving how Vista interacts with Microsoft Xbox 360 games consoles.

The fourth update patches Windows Media Player 11, the default audio- and video-playing software included with Vista. Microsoft offered few details but the company's spokeswoman said that the 8.8MB download for the 32-bit version of Vista "eliminates corruption of Media Player database in certain scenarios and of media stream in certain scenarios."

The updates will be distributed via Windows Update "in the near future," the spokeswoman added, but as in August, she would not pin them to a date. Microsoft's next scheduled Windows Update releases are due out next Tuesday.

Microsoft has aggressively promoted its ability to update Vista through Windows Update, even going so far as denying after Vista's launch that it needed to produce a comprehensive service pack. It has continued to claim that most issues can be addressed by Windows Update.

Mike Nash, the Microsoft executive who leads Windows product management, was the latest to brag about Windows Update's prowess in keeping Vista running smoothly. "The really important updates we can release with Windows Update, and the need for a service pack is actually reduced," he said.