There are several forms of malicious software infections, (malware), which may compromise your computer system. Unlike viruses in the old days, most malicious software operates in the background without the computer user even knowing that they are there. Among them are:

Adbots –
Adbots are used to deliver unwanted advertisements to your computer, usually in the form of pop up windows. While these are not that harmful in and of themselves, they do indicate that your computer is not adequately protected and is probably infected with other malware.

Backdoor –
A backdoor is a way in to your computer, bypassing the normal authentication procedures. Backdoors allow hackers to utilize your computer for any reason in which they can conceive.

DDoS Zombie –
One type of virus you might have on your computer creates a backdoor in to your system for which a hacker can "call up" your computer to be used as a Distributed Denial of Service tool. Basically, your computer is used along with thousands

of other infected computers to send mass amounts of network traffic to a target machine, (usually a web service for which the owner is being attacked for not giving in to extortion). Again, especially with computers connected to high speed internet connections such as DSL, the owner of the computer is likely to not even know that their computer is being used as a tool to carry out malicious internet attacks. A computer which is infected in such a way is said to be a "zombie" computer.

Dialers –
Have you ever had your computer modem dial the network by itself, as if it had a mind of its own? If so, you probably had a Dialer infection on your computer. Besides being used as a tool to automatically connect to sites to download even more malware to your computer, a Dialer can call 900 numbers which can run up your phone bill. Dialers take advantage of the fact that many users save the user and password pair for their dialup connection out of convenience. As it turns out, it's convenient for hackers as well.

Downloaders –
This is malware which is used by someone to download files from your computer to their computer. For example, they might be interested in files with extensions common to spreadsheets, database files or income tax files, to name a few.

Hijackers -
Hijackers take control of various parts of your web browser, including your home page, search pages, and search bar. They may also redirect you to certain sites should you mistype an address or prevent you from going to a website they would rather you not, such as sites that provide software updates. Hijackers most frequently target Internet Explorer.

Keyloggers –
Anyone who has ever had their identity stolen and suffered financial loss would tell you to pay close attention to the following information. Keyloggers are malware which can capture the keystrokes from your keyboard and save them to a file to be transmitted to a hacker later on. Usually, Keyloggers are awakened when you invoke a secure session, such as SSH or ordering products online through a "secure" order form, (so much for 'secure' network ordering). A hacker can gather useful information such as user/password pairs, PIN #'s, account #'s, credit card information and other personal information about you which could be used to steal your identity for the purposes of their financial gain in your name and your liability.

Spybots –
Not to be confused with the anti-spyware software by the same name, spybots send information from your computer back to interested parties about your activities, (such as what websites you visit), for the purpose of gathering information about you.

Spyware –
Spyware is a generic term for malicious software which ends up on your computer, and is used to gather information about you and other files on your computer. This information includes what sites you visit, or in worse cases, personal information used for identity theft.

Trojans –
Simply put, these are backdoors in to your computer in which access is gained by hackers on the internet to either gather information from your computer or to use your

computer as a tool for malicious activity such as internet extortion schemes, sending spam from your computer or using your computer to hack in to other systems.

Viruses –
In the old days, the fact that you had a virus on your computer would usually become immediately obvious as they would do malicious damage to your computer. These days, the creators of viruses have other intentions in mind. A virus has become a generic term for many types of malicious software installed on your computer. The most common type today are Trojans, which like the name implies, create backdoor entry points in to your computer. Especially with high speed connections like DSL, your computer may be compromised and not give any signs of infection.

Worms –
Worms can replicate themselves from one machine to another without the need of downloading them from the internet. They often send themselves as attachments in emails they generate from their infected host computer. Worms can be used as Trojans to open doorways to your computer or to send out spam. You know all that spam you get that is annoying beyond belief? Chances are, most of it is coming from computers on the network which are compromised by worms that remain undetected.