The upper assembly on a Luger P08 (also known as Pistol Parabellum) can still fire when it is removed from the lower section. Never assume that the upper is not loaded when removed from the lower, people have been killed because of this.
Be sure to follow all disassembly and assembly instruction found in the technical section of http://www.lugerforum.com.
If you are not sure of what you are doing or if the gun is not working properly, take it to a competent gunsmith.
Be sure to follow all NRA firearms safety recommendations for your weapon.