There are two kinds of computer cleaning: Internal cleaning and external cleaning. Internal cleaning is the hard disk cleaning. You will clean things that are not physically there but when you run an operating system you can see that all those things are actually there.
Let's start with Internet cleaning. You can delete all your files manually or you can use a software specially programmed to do exactly that for you. CCleaner is the best I know so far. It's free, and yes it has 2 c's in the writing. It's not a mistake. CCleaner will clean your hard disk free of temporary files that can be GB's of files in most computers. Windows creates a folder where will save all the programs it uses there. When you need those files instead of going to the original location that takes time time to find, windows will go directly there. It's much faster. The hard disk loses always some miliseconds and at the end of the day it can mean several minutes, not to count all the stress you will have when you wait for it.
Next thing to delete is all the registry errors. Those errors can actually damage your computer because you think you have the computer in shape when in reality you have all your computer full of trash. The registry is a very sensitive area and you will have to save a copy of it... just in case which is rare. Never happened to me.
Next thing to clean is all the programs you do not use anymore. CCleaner can do it do it for you as well, you can even delete more than one at a time. With windows you will not be able to delete them all at once. Just keep in mind you will need to run the cleaner again because of the temporary files. In the end you will have to go to the folder that has your name and search inside it for the name of the software you just removed and then manually remove that folder once for all!
Finally we have the external cleaning. You must keep your computer out of the dust else you risk having your computer burning because the fans cannot work properly and any electrical charge can set you and your home on fire! First thing to do is to turn your computer off. Then you need to open it, when you do that, you need to remove all the fans and then just use a good dust catcher to get all the dust and the particles you cannot see, trust me you will not be able to do this with a regular rag. Once you do this you will notice your computer running not so hot, which is goof overall.
And this makes the end of this article. Just don't forget to close your computer. Making it run while open was never a good idea...