There's a sequel to that...
A friend of mine bought a very nifty little Sony laptop (cost her a lot of money). It worked superbly for a few minutes, then gradually slowed to a crawl. She put up with this for a year or more, then finally asked me to take a look at it.
The CPU was getting to max temp, then switching into "low speed" to protect itself. Some very simple diagnosis proved that the fan wasn't running. I assumed this was due to fluff etc, and from experience decided that if we were opening up the laptop, we'd replace rather than clean the fan.
A new fan was bought off the web, £10 or thereabouts including delivery. On the appointed day, we operated on the patient. It took a lot of dismantling, but happily the complete instructions for replacing the fan were available on the web.
When at last we got down to the fan, we found ... it wasn't plugged into the power connector on the motherboard. No new fan was needed, and her fine Sony laptop is now performing like a good 'un. She has a spare fan, against the day cat fur and cobwebs bung up this one.