The case, obviously, houses the PC's innards. There are three main case types - desktop, mini tower and full tower.

A desktop case is rectangular a little like a VCR (remember those?) and users would often place the monitor on top. However, they are now generally out vogue in favour of upright or tower cases although many Media Centre PCs are desktop in design. One of the main drawbacks of the desktop is that they can be cramped inside which limits the amount of expansion cards they can hold.

The majority of modern PC cases are tower systems and stand upright. Many people put them on the floor under the desk although some cases are smart enough to stand on top of the desk and be considered furniture!

Smaller cases - often called mini towers - can't hold as much as larger models, naturally. This may not be an issue if you're sure you won't want to expand your PC but such cases may not allow you to add additional disk drives or plug-in large expansion cards, for example, especially the latest generation of graphics cards which can struggle to fit inside a normal tower case. So you need to check a case's capacity if you're planning to add upgrades.

The motherboard in a tower case is usually mounted vertically on one side giving you easy access to components. If you enjoy upgrading or want to build your own PC, you'll want a tower case. But be aware that they are not all created equal by any means. Apart from internal capacity, they vary enormously in what components they come with (fans and so on) and in aesthetics.