There are a vast number of threats that could affect your computer today:
This malicious program category largely exploits operating system vulnerabilities to
spread itself. The class was named for the way the worms crawl from computer to
computer, using networks and e-mail. This feature gives many worms a rather high
speed in spreading themselves.
Programs that infected other programs, adding their own code to them to gain
control of the infected files when they are opened. This simple definition explains the
fundamental action performed by a virus - infection.
Programs that carry out unauthorized actions on computers, such as deleting
information on drives, making the system hang, stealing confidential information,
etc. This class of malicious program is not a virus in the traditional sense of the word
(meaning it does not infect other computers or data). Trojans cannot break into
computers on their own and are spread by hackers, who disguise them as regular
software. The damage that they incur can exceed that done by traditional virus
attacks by several fold.
Software that collects information about a particular user or organization without
their knowledge. You might never guess that you have spyware installed on your
Potentially dangerous applications include software that has not malicious features
but could form part of the development environment for malicious programs or could
be used by hackers as auxiliary components for malicious programs.
Utilities used to conceal malicious activity. They mask malicious programs to keep
anti-virus programs from detecting them. Rootkits modify the operating system on
the computer and alter its basic functions to hide its own existence and actions that
the hacker undertakes on the infected computer.