If you've never had the PC open before, vacuum out all the dust and clean the fan at the same time. It stops the PC overheating and you may see a vastly improved performance.
Removing the dust is always a good idea, but never vacuum inside the PC as you run the risk of building up a large static electricity charge that can damage the delicate parts of the PC inside. Yes, in years past I have used the house vacuum on occasion, but I have been lucky that I did not damage anything.
There are specfic vacuum cleaners that are designed for PC cleaning...although all I can find are blowers for PC on Amazon, so I don't know how expensive they will be.
See here for a safer methods (either use blowers/compressed air to push the dust out or use the vacuum a distance away from your PC as you use brushes - that also don't build up static, so check the materials):
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