This procedure will tell you how to relocate your PC from your desk to a position on the floor. This is an enhancement that you can use to free up desk space and make your PC quieter at the same time. See here for details on this optimization , including its advantages and disadvantages.
Tower cases are much easier to put on the floor than desktop cases.
Some desktop models can also be put there, but things are more complicated here. You need to be careful because some desktop units are not square on the sides and will topple if mounted sideways. I recommend using a tower case for this reason. Find a spot for the PC on the floor. You want the PC to be somewhere where it can be secured physically to ensure that it won't fall.