Question: My CPU cooler is too big so i cant get my motherboard out to clean it it's like 7 years old and I need to get the dust out. I have tried to take it out without taking off the motherboard but it doesn't fit.

Answer: 
**That is a bad idea! Continue reading...**

You need physical contact to move heat efficiently from the **CPU** heatspreader to the heatsink. The role of **thermal paste** is to "fill in the gaps" and allow for better transfer of heat from the heatspreader to the heatsink.

**What will happen if I don't put a thermal paste in my processor and heatsink?**

Things don't work as efficiently as they should. Your CPU's operating temperature will be higher. It may need to slow itself down (thermal throttling) to stop from overheating and failing. 

Does this mean that your CPU will burn out and die? No. Does it mean that it is more likely to? Yes. Does it mean that your CPU will throttle? No. Does it mean that it is more likely to? Yes. Can you run your CPU without thermal paste for years and still have an operating computer? Yes. Is it a good idea to try? No. 

The truth is that properly applied thermal paste is an important component of a cooling system and going without it is a *bad idea*. It's also a good idea to service your CPU every now and then (once every year or two) and replace dried and cracked thermal compound with a fresh application. If you find that thermal paste wasn't applied to your CPU when it was assembled, that's bad, but it isn't the end of the world. Modern CPUs will attempt to shut down before reaching a temperature at which damage occurs, and before that, they will slow themselves down.
Yes, you should replace the compound if you remove the heatsink.

Thermal paste is generally either a near-fluid or putty like compound when applied. Over time it "sets" into a solid due to heat and forms an effective seal between the CPU case and heatsink.

When you remove the heatsink you break that seal and create gaps in the compound. As the compound is no longer malleable it cannot reflow into gaps when you refit the heatsink and can easily end up with air bubbles and poor contact between the CPU and heatsink.