Microsoft Corp. posted a major package of updates and security fixes for its Windows Vista operating system for download starting Tuesday.

People whose PCs run the newest Microsoft operating system can visit Microsoft's Windows Update or Download Center websites and download the free Service Pack 1.

The update to Microsoft's Windows Vista will block some programs from running for 'reliability reasons.'

Many of the fixes have already been released as part of regular monthly updates in the year since the operating system went on sale to consumers. Microsoft has said SP1 improves Vista's reliability, security and performance.

Some early testers of the service pack have noticed improvement in performance when copying files over a network and playing video games that weren't designed for Vista.

So far, the software maker has determined that only a handful of programs will fail in some way after SP1 is installed.

Microsoft said SP1 will block several applications from running for "reliability reasons."

The list includes BitDefender Antivirus and Internet Security, version 10; Fujitsu's Shock Sensor hard drive protection for rugged laptops; two versions of Jiangmin KV Antivirus software and Check Point Technologies' Zone Alarm Security Suite.

The company said a few programs won't run on SP1, such as web application design program Iron Speed Designer, while others will stop working well, like The New York Times Reader application.

Web retailer Amazon.com Inc. is selling boxed copies of Vista SP1 for shipping Wednesday.

Redmond, Wash.-based Microsoft said it will push out the upgrades in mid-April to computer users who have set their PCs to receive automatic updates.