Perpetual motion is impossible because
1) every entity needs to be produced, fitted, started. The act of starting means that the motion is not perpetual backwards (so no infinite backwards motion)
2) every entity is subject to the laws of thermodynamics. Every system (on any scale) gets more disorganised as time passes (known as entropy). Any machine will eventually fail because it's parts will fall apart or break (or the elements in it will, for example, react with, say, air, and rust - or even just be prey to nuclear decay). The timescale may be long, but "perpetual" implies forever.
3) Power is needed. A perpetual motion device is assumes to provide its own power. This again is impossible. There is always a loss of power when any action is performed, and it is iot possible to regain 100% of this energy. you could build a secondary device to provide the remaining energy - which means that you need external power, which will not be perpetual.
Here is an interesting read on the subject:
Physics 101 For Perpetual Motion Inventors
http://www.lhup.edu/~dsimanek/museum/phys101.htm
