PC getting slower and hotter CPU

My PC has been getting slower crashing to desktop in intensive games such as Battlefield 3.
I rebooted and connected a CPU and too hot message on reboot.
And next I removed the case and blew out and cleaned dust in that and the CPU too. But the result was worse.
Next, I installed HWMON and surprisingly temps approached to 100c.
What would be the cause for this?
