Q. What is UAC (User Account Control) in Windows Vista?

When a member of the local Administrators group runs a normal task, it is performed without administrative privileges. When the task requires administrative privileges, Windows Vista halts the process until you acknowledge the following prompt:

If the user is not a member of the local Administrators group, a prompt like the Windows XP Run As command asks for valid administrator credentials.

NOTE: See How can I disable UAC (User Account Control) in Windows Vista?