You cannot launch something as admin if you're not admin yourself. It is 100% impossible, unless you count zero-day exploits. The right-click 'run as admin' also pops a user/pass dialog. If you're using UAC on Vista or higher, and you're already an admin user, this box is replaced by a simple 'Yes/No' popup. But it's still the same thing.
To run an exe as admin, try searching the nsis forums for 'run as admin' for example, or just use runas.exe.