How you use your computer and how often you use it will dictate what you have to do to clean it. Unlike other people, I believe you only need to clean it when the grime starts to bother you.
The dummies website has a pretty good article on how to clean a laptop computer. It seems to be a bit anal about it and in some cases, that’s a good thing.
Some of the techniques used to clean a laptop computer can also be used for a desktop computer. Some things can be a lot simpler. Before you do anything, make sure your computer is off and unplugged.
If you work in subdued lighting, like I do, you won’t notice the dust and fingerprints so much. That all changes when you get out into the daylight. It all suddenly becomes extremely visible and annoying.
A damp cloth, wet with only water, will take care of almost everything. It won’t remove the insects that get under the screen (I have two insect skeletons under this one). The best thing for that is pulling the screen toward you with a strong suction cup.
If you think you need to use something stronger than water, think again.
It depends on the keyboard. The keyboard component on modern laptops doesn’t have any gaps where debris can get under the keys. You’re not that lucky if you own an old one or you’re using an inexpensive keyboard with a desktop.
Don’t use canned air. It may seem to be the easiest thing to do, but that air will blow debris into places where you probably don’t want it.
A keyboard vacuum works well, if you can find one. I like to use a small, clean paintbrush to sweep everything off the keyboard.
With an inexpensive desktop keyboard, I usually just turn it upside-down and shake out the debris. If I have a problem with keys not working right, I just replace the keyboard. I can get one locally for less than $5 USD. One brand, made in China, is about $2 USD.
The areas where I rest my hands is where it gets the dirtiest.
I’ve become a hand-washing fanatic. After scraping hardened skin oil off the “palm rest” area more than a couple of times, I figure it’s easier just to keep my hands clean.
There’s a reason I mention the case last. You’ll need to clean the monitor screen the most often, followed by the keyboard area. The case is like an after-thought.
As I mentioned before, use a damp cloth for cleaning. Don’t use anything stronger than water. Purified or distilled water is better than tap water. Tap water can contain more chemicals than it should, especially chlorine, which can be corrosive.
The average MTBF (maximum time before failure) for most computers and their components is five years. I have monitors in storage that are much older. The only reason they’re still good is because I cleaned them as rarely as I could and only with a damp cloth.