Can anyone of you tell me how can I now get my 8500's performace up again?

Locate the whole metal cooling assembly at the lower right end of the motherboard that is positioned over the 8500's processor. Remove all the four screws securing the cooling assembly to the motherboard. Lift your cooling assembly upwards and then to the right to remove it from your motherboard. Place the end of this flat headed screw driver into a locking screw at the right edge of the processor. Turn on your screw driver counterclockwise until your processor unlocks from the mother board socket. Grab two different type of edges of the processor and then pull it off the socket. Place your new processor unit over your socket. Line up the pins on your underside of the processor with the holes on the socket and then push your processor down. Turn on the locking screw clockwise with your flat head screwdriver to just lock the processor. Put it back together each of the parts in the order that they were removed. Turn on the Dell Inspiron 8500.