Vista - third party software updater

Forgive me if this has been asked thousands of times but I've done searches
and haven't found an answer.

We have an application that has it's own updater that works by going out to
a local server (not the internet) on the network and checks for updates each
time the program starts. If an update is found the files are copied over
and installed using a .inf file and win32 setupAPI calls. Some of these
files are programs and some are data. In Vista are administrative
permissions required for the setupAPI calls to complete successfully? I
know they are needed for the initial install, but what about for updates?
Also, is it different when the calls are overwriting existing files as
opposed to creating new files (like a new .dll for an executable that is
overwritten)?

Thanks in advance,
jim