Microsoft on Tuesday made the first major update to its Windows Vista operating system widely available for download.

Computer users with Vista already installed on their systems can now download Vista Service Pack 1 for free from Microsoft's Windows Update site. Some retailers, including Amazon.com, are offering boxed versions of Vista SP1 for sale starting Wednesday.

The software is available in five languages: English, German, French, Spanish, and Japanese.

Microsoft recently made Vista SP1 available to some developers and commercial users as a download from its business Web site, but the software has not been publicly accessible until now.

Vista SP1 contains numerous features designed to enhance the operating system's speed, performance, and stability. Among other things, it offers a patch that will allow users to run the BitLocker encryption tool on multiple hard drives. It also improves the speed at which the OS wakes up from "hibernate" mode.

SP1 will also remove from Vista the so-called Kill Switch -- a feature that deactivated key components of the OS if Microsoft detected users were not running a properly licensed copy of Vista.

The feature was plagued by false alarms that flagged thousands of legitimate Vista users as software pirates.

All told, Vista SP1 includes more than 300 hot fixes for the OS.

Users will need to install three prerequisite files onto their computers before they can upgrade to Vista SP1. Those files are also available through Windows Update.

Vista SP1 will not install on computers that use peripheral device drivers which Microsoft has deemed incompatible with the service pack. The list includes a small set of audio and display drivers made by Realtek and Intel, as well as drivers from several other manufacturers.

Microsoft said it's working to resolve the compatibility issues.

Amazon's Web site shows that both the full and upgrade versions of Vista SP1 are currently available for pre-order and will ship on March 19th.

It's expected that a number of retailers in addition to Amazon will start offering Vista SP1 this week.

Microsoft is hoping that Windows Vista SP1 will quell some of the disappointment that greeted the operating system's initial rollout early last year. Many corporate and home users complained about its resource requirements and lack of compatibility with existing applications.