There's no surprise in it for me; Trojans are nearly a most aggressive type of malware that ever hit the Web in the last years. Those who have been in IT industry for a long time remember that first computer viruses were created mostly for fun – just to make sure “it really works!” Nowadays the reasons why Trojans are so widespread and so hard to remove are different: making money and taking control over infected computers.

It's hard to remove Trojan virus because of three main reasons:

Trojans self-replicate and continue self-distribution from infected computers. An address book in email client is a good place for a Trojan virus to start sending itself to other networked computers;

Because there's money to be made in malware industry, new Trojan virus horses are created with special accent on ability to fight antivirus applications. In fact, any security program that largely depends on definition updates is a sure looser in this war; it's next to impossible to remove Trojan virus with an outdated antivirus that lacks strong heuristics algorithms;

The diversity of this type of malware challenges the whole security software industry. Those who used to call themselves No.1 on the market, are easily knocked off pedestal by Trojan virus creators.

This is one of the reasons why tools developed by volunteer programmers and small software companies have gained so much appraisal from owners of infected computers. One of such trojan virus killers is called SmitFraudFix and is meant to remove a number of fake security applications, installed with the help of sneaky malware. For some of the nastiest pests there are even more targeted programs developed, i.e. for Vundo and Zlob malware – programs that guilty of millions infected computers.

A software company a-squared also joined the movement of free Trojan virus killers by releasing a rogueremover. Besides, the company created a command-line scanner to help system administrators and advanced PC users in winning the Trojan war. Though focused on rogue security software, it targets a number of threats. Malwarebyte's offers its free anti-malware, and so does SUPERAntiSpyware.

Online scanner is another common means to get rid of a trojan virus for free. Most major antivirus companies offer web services, and some provide integrated scans with engines from several manufacturers.

However, the availability of free tools that have gained reputation and popularity thanks to their elaborated algorithms used to remove Trojan virus infections doesn't mean it's easy to clean out a contaminated system. This type of malware is pretty tough to get rid of, and often a series of scans is required to erase the malicious files and registry entries left by Trojans.