You can write a LV program which is installed and loaded from setup.exe. This program will ask for the password, etc, and if it is accepted, the program will install the main program. You could hide the main program setup.exe by changing the name from setup.exe to *.* manually, and then when the password is accepted the program can change it back to *.exe. You can run an executable from LV by using the System exec.vi in the communication section in the Functions palette.
I know it sounds a bit long-winded, but it's all i can think of at the moment, and i'm sure it would work, but if anyone has a better method then i look forward to hearing it too!
Kim