I have a Media Center PC. For my sins, I really like it.
However for the past few (well many) months it has been failing to power manage correctly, never sleeping and running the fan all the time. It has also been running like a dog.
Being the clever computer type I am, I immediately diagnosed a bad case of Windows Rot and tried to make time to reinstall it.
Then we moved house and I noticed it was filthy. Every vent at the rear was choked with vacuum-cleaner-bagesque dust bunnies.
Having administered a quick stiff brushing, it is now running cooler, quieter and faster.