1. A boot sector virus stays resident by infecting the boot sector of the computer
2. A Master boot record (MBR) virus infect the first physical sector of all affected disks
3. File viruses either replace or attach themselves to executable files, and most commonly found virus.
4. Macro virus attaches itself to documents in the form of macros.
5. Memory viruses are viruses that execute and stay resident in memory. Trojan Horse is an example of memory virus.
A trojon is not a virus. The principal variation between a Trojan horse, or Trojan, and a virus is that Trojans don't spread themselves. Trojan horses disguise themselves as valuable and useful software available for download on the internet. Trojan may work as a client software on your computer communicating with the Trojan server over the Internet.