Explanation:

- Tower model: This is a type of personal computer designed to stand upright and house all components within a single vertical case. Commonly referred to as desktop computers, these are widely used for personal and professional tasks.

- Notebook: Essentially a lightweight version of a laptop, notebooks are portable computers that can be easily carried around. They offer similar functionality to desktops with added mobility.

- Laptop: Laptops are fully-fledged personal computers designed for portability. They integrate all components, including the keyboard and screen, into a single unit that can be folded closed.

- Hand-held computer: Also known as PDAs (Personal Digital Assistants), these are compact devices designed to be held in one hand. They often do not possess the full capabilities of PCs but can offer basic computing functions.

Hand-held computer is not typically classified as a personal computer in the conventional sense.