A Trojan (or Trojan horse) is a malicious program contained inside apparently harmless programming. It can get control and do its chosen form of damage, such as ruining your hard disk or allowing someone to take over your computer remotely. In one case, a Trojan was in a program that was supposed to find and destroy computer viruses. A Trojan may be found by antivirus software, but not always.

A Trojan can also be put onto our computer if we go to a malicious Web site. When the site is loaded, a series of pop-under pages can be loaded by another Web host, and one of those pop-unders can download a file to our computer. Once it gets on our computer, this particular Trojan redirects search engines such as google.com and altavista.com to a different search site. If your computer is infected, you may not even realize it. One clue is when a whole bunch of pornography and gambling popups start appearing.

Trojans can pose a privacy risk, because they can get a list of what pages we have visited. The information we type into the pages can be captured, too, and that could allow a lot of sensitive information to get out. Many of the newer viruses and worms have a trojan component that is designed to open up a communication port on your computer so that someone can get access to all of your information without you even knowing it.

Although the antivirus programs work to keep up with ALL of the nasties that can get on your computer, a program that specifically looks for Trojans is a good tool to have. a² (a squared) is an excellent tool to use for an in-depth scan of your system. The free version is available from EmiSoft. This program contains only the basic scanner; Background Guard, Automatic Updates and other advanced features are only available with a² Personal Edition.

One of the more annoying trojans is a homepage hijacker known as CoolWebSearch. What began as a simple-to-remove problem has grown in complexity, and the difficulty of removing it has grown along with it. SpywareInfo has a great article on the history of this trojan located here (warning - lots of geek stuff in this article). More importantly there is a removal tool located here.