What are the main types of computer viruses?

Well, in general there are three different types of viruses: Worms, Trojan Horses, and regular generic viruses. Worms are malicious programs (they arn't really considered viruses because they don't need to attach to a program) that modify or delete files on your computer and replicates itself to send to other users. This means that if you are infected with a worm, all the members of your network or email list (unless you have a good antivirus program) are suseptable to be infected. Trojan Horses are programs that appear to be regular programs that help the computer or do nothing, but really cause harm to your files (hence the name). Trojan Horses are not considered Worms because they do not copy themselves, but Trojan Horses can spawn Worms which can. Generic computer viruses are any types of viruses that damage your files by attaching themselves to a program. They can be transferred to another computer, but must be transferred as part of the original virus. There are also other ...  more

System Sector Viruses infect executable code found in certain system areas on a disk. There are boot-sector viruses, which infect only the DOS boot sector, this kind of virus can prevent you from being able to boot your hard disk, and MBR viruses which infect the Master Boot Record on fixed disks and the DOS boot sector on diskettes. All common boot sector and MBR viruses are memory resident. File Viruses infect applications. These viruses usually infect COM and/or EXE programs, though some can infect any program for which execution or interpretation is requested, such as SYS, OVL, OBJ, PRG, MNU and BAT files. File infectors can be either direct-action (non-resident) or resident. A direct-action virus selects one or more programs to infect each time a program infected by it is executed. A resident virus installs itself somewhere in memory (RAM) the first time an infected program is executed, and thereafter infects other programs when they are executed or when other conditions are ...  more