Is the update in the form of an MSI or MSP file ? If so, you could use the /qb! command line option that display a progress dialog.
The alternative method is to create a "wrapper" script which you run to install your program. The wrapper script is run by NS, and it, in turn, launches the update you are deploying. When the update is finished, control returns to the wrapper script, which can then display a "Finished" message using a system modal dialog box so that it appears on top of any other windows. However, all this is dependent on the degree to which the installation agent allows an installer to interact with the desktop.