My Pc has smoke coming out from top,where the fan is. Any idea?

With the limited info in the question and the fact that you say smoke noriced at the top where the fan is indicates there's a problem with the power supply. The p/s has a radial fan and most of the high current wiring is here. Therefore, be careful using the PC because of a fire hazard. Rather just open the side panels and ascertain for certain where the smoke is coming from. The P/S unit is easily spotted.

Another possible source is the CPU which is also fan-cooled and which gets very hot, although protection circuits ought to switch it off before it gets hot enough to smoke.