I'm guessing you have the Pentium D installed or something like a Pentium 4 HT. Not to mention the D series runs hot.
In both cases your CPU is highly outdated & even the simplest tasks is enough to keep your CPU at 100% all the time. Hence, the extra heat.
with all do respect you need a complete new system
You can assemble a brand new PC for less than 300USD if not less
Presuming that you have already verified that the CPU fan is working, this could be one of two issues: (a) the heatsink-fan unit has disengaged from the processor and needs to be reattached or (b) the thermal interface material (TIM) has aged to the point where it is not effective. For both issues, the response should be the same,
- Remove the heatsink-fan unit.
- Remove all TIM from both the heatsink-fan unit and the processor. Use a soft (dry!) cloth; you do not want to scratch either surface.
- Apply new TIM to the processor surface. Purchase good quality TIM, Arctic Silver or something similar.
- Reinstall heatsink-fan unit, ensuring that all four attachment points are holding securely.
- Verify that the fan is working properly after reapplying power.