A clean computer is a happy computer.

I just recently "re-virginized" a friend of mine's Mac Pro. He was seriously thinking about junking it because it was running badly, crashed a lot and was really noisy.
After moving his personal files, etc. off the drives, I did a square one reinstall - we put a larger system drive in the machine, reinstalled and updated the OS and then put back his software.
Total time for that, about four hours.

Then, I went into that noisy computer itself and started cleaning.

It sounds like a very basic thing but it's something that many people don't do. They figure that their house is pretty clean and it's probably not so bad.
Or it's something that they intend to to, but just haven't gotten around to. It's a pain in the next because the computer is on the ground, and a bazillion wires are coming out of the back...

Since I have pretty high end desktop computers that must work perfectly (hey, who doesn't?)
these inconveniences are not an excuse. I have my computer on a rolling dolly (i made it with four casters and a small plywood board) and roll it out enough to get in there and do my process... inside and out every two months.

Put it in your calender as a recurring event. It only takes ten minutes to do and will save you a lot of headache. Make it as much a part of your life as doing laundary, changing bedsheets, cleaning the bathroom or vacuuming the living room.

Get some compressed air. Turn the system off, open it up and blow that dust out.

Heatsinks: if you blow the dust off the fins regularly, that should suffice. If you haven't done it in a long time, get in there with a q-tip or small brush. Scrub. Even the thinnest layer of dust on these fins affects things in a big way.

Fans - obviously all the air flow fans (the big ones) are important, but as or more important are: CPU fans, GRAPHIC CARD fans and of course the PSU (power supply).

Hand turn all the fans to make sure that you haven't dislodged something that has caused the fan to get stuck.
Blow out all the ram, and then the entire motherboard.

If you have open vents, think about getting air-filters for them. You can go to any Home Depot and buy shop vac filters, cut them to size and duct tape them in there. it will go a long way to keeping dust out of the system.

Basic stuff that you've been told a million times.
Now do it.

You will be stunned by how much better, and quieter your computer is.

My friend- He had tears in his eyes when we were done and rebooted his computer. His love affair with it has begun all over again.