The advantages of upgrading the server that hosts your existing IIS Web sites and applications to IIS 6.0 include the following:

•	Reduces the time required to deploy IIS 6.0. Upgrading requires minimal user interaction.
•	Reduces possible configuration errors. Because the majority of the current operating system, IIS, and Web site configuration settings are retained during upgrade, fewer configuration errors result.

The disadvantages of upgrading the server that hosts your existing IIS Web sites and applications to IIS 6.0 include the following:

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Retains previous versions of software. The upgrade process upgrades only software components identified by Microsoft® Windows® Server 2003, Standard Edition; Windows® Server 2003, Enterprise Edition; and Windows® Server 2003, Web Edition. Any other installed software components, such as applications or application dynamic-link libraries (DLLs), remain unchanged. During the IT lifecycle of the server, you can install and subsequently remove many of these additional software components. The software removal process often leaves DLLs, other files, and folders on the system. Over time, these unused components can accumulate, consume available disk space, and potentially cause instability with existing applications.

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Retains the previous registry configuration. The upgrade process makes the appropriate modifications to the registry entries identified by Windows Server 2003. Registry entries are created by an application as it is installed, but are not removed when the application is removed. Cumulatively, these registry entries consume disk storage and can make troubleshooting registry-related problems more difficult because the unused registry entries can have similar naming conventions to active registry entries.

After upgrading a server to Windows Server 2003 and IIS 6.0, the server runs in IIS 5.0 isolation mode, which allows IIS 6.0 to emulate the memory and request processing model in IIS 5.0. However, to take advantage of the security and performance benefits of IIS 6.0, you need to configure the server to run in worker process isolation mode, which is the new IIS 6.0 memory and request processing model.

When upgrading existing IIS Web sites is not the best solution for your existing server, consider migrating your IIS Web sites to a newly installed server running IIS 6.0. For information about migrating your existing IIS Web sites to IIS 6.0, see Migrating IIS Web Sites to IIS 6.0 .