viruses - cannot operate automatically and need users to run the Trojan for it to cause the damage. It is more a case of social engineering concept using human flaws to infect the computer rather than flaws in the system design or configuration. Trojans rely on fooling people to allow the program to perform actions that normally they would not allow.

Damage

Trojans contain malicious code that when triggered cause loss, or even theft, of data.
Trojans contain triggers that can start damage on a particular day or on a particular event.
Trojans are dropped onto computer which make it easy to execute and install other spy ware and ad ware that can compromise the privacy of password and financial transactions
Trojans are used to gain internet browsing habit and are used by hackers and spammers together.

Examples of Damage

Key snatch

This program is for password recovery. It records all the keystrokes and this is sent to the attacker or hacker. Your credit card passwords, email passwords and other sensitive information can be obtained by this means. These are commonly installed on your machine with remote access Trojans.

Dropper

Similarities with installer are what make Trojan droppers dangerous. They run or install a virus. These are not common now days.

MoneyTree

It is an activeX control for downloading dialers. These dialers are for connection to porn sites. These dialers often redirect to other pages or misspelled domains.

MoneyTree.DyFuCA

A MoneyTree variant this installs dyfuca.ocx and dyfuca.inf in the Downloaded Program Files folder. This installs Internet optimizer an error page hijacker for IE.

Unknown Trojan

Many scans come up with this result. The action is like that of a Trojan and many times it is difficult for the Trojan to be removed with a scanner. The best would be to take a back up and format your system.

System Soap Pro

This is supposed to erase all privacy of your work on the computer. This is an advantage as no one can use the log file or any other data from your computer.

Spyster 1.0.19

Trojan that makes unwanted changes on your system and installs many advertising related components like tools bars, search bars etc. These can collect, transmit and share sensitive data.

Trojan.Win32.Revop.c

It is not dangerous but affects users privacy

Coulomb Dialer

It can change your local dialer for the internet connection to an international phone number. This is activated by Active X

Unknown Dialer

These are various types of dialers that change your system and dial into other phone numbers.

Prevention

The best prevention is to load an antivirus and scan once a week. Regularly update the virus definition files of the anti-virus.

Regularly update patches and service packs of the operating system and service packs.

Removal

There are a number of techniques for removal and it depends on how computer savvy you are.

The easiest would be is to download antivirus software’s and run them. Some software’s miss some viruses and Trojans while some others find them. It is advisable to have some good antivirus software’s.
You could utilize online scan functions of many antivirus software for detection and removal.
In case of being an advanced user you can find out where the Trojan has attacked and follow instructions that you find given by experts on many forums.

Trojan horse virus is dangerous and the method is only safe computing and vigilance

Some Viruses can Crash you PC. Check out the Types of Viruses
Beware of Email Viruses in Spam - Top Tips to Stay Safe
Popular Email Viruses and the Effect They Had on the Internet
Macros: Better Applications or Worse Viruses