PC keeps shutting down and saying it is overheating.

Hi all
I have just taken the cover off my PC and cleaned out all the dust.I took the heatsink off the processor to clean it and re attached it. Now my PC keeps shutting down and is saying it is overheating .The fan is on constantly .what have i done? any help as soon as will be good.

You should reseat the CPU HSF and apply a fresh coat of thermal paste onto the CPU, first removing any of the old, dried out paste with isopropyl alcohol (rubbing alcohol). A very light coat of paste will be enough.

The fan-heatsink assembly must be seated with properly applied thermal compound.It's also possible that the fan itself is dying, you may need to replace it or buy a better fan.