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 use Microsoft’s Windows Update tool or visit its Download Center Web site and download the free Service Pack 1. In some cases, computer users may need to download older updates before they’ll be able to install SP1.

Many of the fixes contained in SP1 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.

Before SP1 was made widely available, Microsoft had determined that a handful of programs will fail in some way after SP1 is installed. On Tuesday, a Windows team blog said PC users with some drivers installed will “temporarily” not be able to get SP1 at all.

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.

Certain device drivers from RealTek AC, Intel and Symantec are among those Microsoft said would prevent an upgrade to SP1. The software maker said PC users can seek out updates from most of the makers of those devices to fix the problem.

Industry analysts offered mixed reports on whether Vista SP1 makes a noticeable difference on the way their computers run.

Michael Cherry, of the research group Directions on Microsoft, said that after installing SP1, the time…