Umm, that isnt a melted cpu....
That metallic stuff around the core is just whats left of your thermal compound...
A 700mhz celron couldnt produce enough heat to melt a core.
Liquid nitrogen isnt that great as a cpu cooling tool. It may be extreamly cold, which is good for a short ammount of time, eventually it will warm up and getting rid of the heat in the LN qould be rather problematic.
The only useful way of doing it, would be to just keep adding more liquid nitrogen, which is how the supercomputers do it.
Overheating stuff is definatly fun tho