Windows computer intruders often use three tools to gain access to windows based systems. BackOrifice, Netbus and SubSeven . Once installed these back door programs allow other people to access and control the computer where they are installed. Intruders can change system configurations and also infect the computer with a virus. A Trojan Horse is a program that created and installs a version of system utility that copies system information or hides it. Trojan horses trick users into installing Back Door programs.

Viruses and other types of malicious code often spread as attachments to e-mail messages. Often , the Viruses or malicious code are distributed in amusing programs.

A worm is a self copying virus that spreads very fast often using the contacts in Outlook. An example of such a worm which wreaked havoc was the VBS /LoveLetter worm. It contained a attachment in mails called LOVE-LETTER-FOR-YOU.TXT.vbs. Another Two famous examples of worms are the MS-Blaster and Sasser worms.

In cross site scripting , a script is attached to something sent to a website, such a URL, an element in a form , or a database enquiry. Later when the website responds , the code is transferred to the browser

Internet chat application provide ways of exchanging information between computers without any hindrance . Many chat clients even have options of transferring files to and from without any authentication or scan.

A denial of service attack causes the computer to crash or become so busy that it becomes impossible to work on it. In addition the computer might participate to be a mediator for such an attack for the whole network.

Spyware is the new term for advertising supported software (Adware). Advertising in shareware products is a way for shareware authors to make money, other than by selling it to the user. There are several large media companies that offer to place banner ads in their products in exchange for a portion of the revenue from banner sales. If the users find the banners annoying, there is usually an option to get rid of them by paying the licensing fee. Unfortunately, the advertising companies often also install additional tracking software on our system, which is continuously using our Internet connection to send statistical data back to the advertisers. While the privacy policies of the companies claim there will be no sensitive or identifying data collected from our system and that we shall remain anonymous, the fact remains that we have a server sitting on our PC that is sending information about us and our surfing habits to a remote location using our bandwidth. Spyware has been known to slow down computers with their semi-intensive usage of processing power, bringing up annoying pop-up windows at the most inappropriate times and changing our Internet browsing setting such as our home page or default search engine to their own services. Even if many do not consider this illegal, it is still is a major security threat 

The list goes on. But Security measures can be implemented to ward of most security threats.