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A computer can be given a quick once-over with a vacuum or dust cloth but it's a good idea to give it a more thorough cleaning occasionally.
To clean a monitor, first unplug the power supply, then lightly dampen a clean, soft, lint-free cloth with water. Wipe the screen gently with a back-and forth motion. Wipe the display case with a non-abrasive, soft, dry cloth to pick up dust.
To clean the keyboard, turn it upside down over a wastebasket and give it a good shake. Then vacuum it with your brush attachment. To clean the keys, rub them and the surrounding with a microfibre cloth.
You can also use a can of compressed air available at camera shops.