Summary 
Windows Installer version 4.0 complies with User Account Control (UAC) in Windows Vista. With authorization from an administrator, the Windows Installer can install applications or patches on behalf of a user that may not be a member of the Administrators group. This is referred to as an elevated installation because the Windows Installer makes changes to the system on behalf of the user that would not normally be allowed if the user were making the changes directly.

More Info
This knowledge base article from MSDN currently discusses five major aspects of Using Windows Installer with User Account Control. Information and links referring about designing applications for managed systems for Vista, Application Information Service (AIS)* in Windows Installer 4.0, Installing a package with Elevated Priviledges for a Non-Admin, Patching per user managed application etc are some of the key contents you will discover in this article, very important knowledge for long awaited Windows Installer 4.0 and Windows Vista.

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*Application Information Service (AIS): A system service of Windows Vista that facilitates starting installations that require elevated privileges to run. Provides the Consent UI used by User Account Control to prompt a user for administrator authorization.

Source: MSDN