UAC can be a rather complex concept to wrap your head around. Generally speaking, a child process inherits its access token from the parent process. However, this only occurs if both processes have the same integrity level:
Each application that requires the
administrator access token must prompt
the administrator for consent. The one
exception is the relationship that
exists between parent and child
processes. Child processes inherit the
user access token from the parent
process. Both the parent and child
processes, however, must have the same
integrity level.
Integrity levels depend on a variety of things, but generally speaking, a web browser is a low integrity application, and will likely require an additional UAC prompt if it tries to do any operation requiring a higher level of privilege:
Windows 7 protects processes by
marking their integrity levels.
Integrity levels are measurements of
trust. A "high" integrity application
is one that performs tasks that modify
system data, such as a disk
partitioning application, while a
"low" integrity application is one
that performs tasks that could
potentially compromise the operating
system, such as a Web browser.
Applications with lower integrity
levels cannot modify data in
applications with higher integrity
levels.
If you wish to learn more about UAC, the following articles are a good resource:
