To understand the differences between a virus, worm, and Trojan horse, it is important to consider the following:

1. A virus is a type of malicious code or program that, once executed, replicates itself by modifying other programs and infecting them with its code.
2. A worm is malware that replicates itself and often uses a computer network to spread to other computers. It does not require a host file to spread.
3. A Trojan horse is a malware that carries another type of code, and once executed, it can give the attacker access to the computer or allow the installation of further malware.

To help secure your computer from viruses, worms, and Trojan horses, there are several steps you may take:

1. Keep your operating system and software up to date.
2. Scan your computer regularly with anti-malware software.
3. Be wary of suspicious emails and exercise caution when downloading files or clicking links.
4. Only download software from websites that you trust.
5. Back up important files on a regular basis.