Consumers will be able to purchase the first major update to Microsoft's Windows Vista operating system starting on Wednesday, if Internet retailer Amazon.com is to be believed.

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.

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 widely available to date.

It's expected that a number of retailers in addition to Amazon will start offering Vista SP1 this week. Microsoft will also likely make it available as a public download in the coming days. The download version is free to users already running a licensed copy of Vista.

Vista SP1 contains a number of 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.

Vista SP1 itself isn't without problems, however.

Microsoft has yet to finish ensuring that the service pack will work properly with the thousands of models of keyboards, printers, mice, and other peripheral devices commonly attached to personal computers.

Such peripherals use bits of software called drivers to communicate with the host computer.

The problem is that Vista SP1 won't install some device drivers correctly. Microsoft says the issue is confined to "a small number" of drivers and that it's working on the problem.

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.