A Trojan is a type of computer virus that has been known to cause your computer to crash and being unable to function properly. It slowly deletes files and programs from your computer and alters the manner in which your desktop will appear until such a time that your computer’s operating system can no longer operate due to some of the crucial programs and files needed have been deleted from the computer’s hard drive.
The Trojan computer virus was named after the mythical Trojan horse which led to the fall of Troy. According to the myth, the Trojan horse was delivered as a seemingly harmless gift and was placed in the center of the city. Unknown to the inhabitants of Troy, the horse contained an entire army. As soon as the entire city was sleeping, the army came out of the horse and seized the city by surprise. This is the same way how the Trojan virus works. It is usually sent to a computer in the form of an email attachment. Once this attachment is opened, the virus program is able to infiltrate your computer hard drive and would slowly start doing its damage.

Here are a few tips to help you prevent your computer from being infected with a Trojan computer virus.
Only open emails from trusted sources
While there are many web-based email servers that provide a scanning feature to screen email attachments prior to downloading, users of Microsoft Outlook and other similar email browsers are not able to provide this feature. If you are unsure of the sender of the email, it is best to simply delete this email without opening it no matter how inviting the subject may be.
Update your anti-virus regularly
Anti-virus software programs have been developed to block any threats to your computer. Running your anti-virus regularly is not enough to ensure your computer hard drive is protected. This is because anti-virus software programs would only detect different kinds of malware, including Trojan viruses, which are stored in their databases. As such, it is important to first update your anti-virus database prior to running this to ensure that it is updated with the newest malware information so that it would be able to detect even the most recent kinds of malware roaming around cyberspace.
Get a software program designed to detect spyware and Trojan viruses
With the proliferation of computer malware over the Internet, there are now programs that have been designed to detect spyware, Trojan viruses, adware and the like which are often not detected by most anti-virus software programs. While there are many of these programs available for download over the Internet free of charge, make sure that you only download these from reliable and trusted websites since some computer malware have been designed to appear as computer anti-virus programs.
Trojan droppers are very dangerous they are hard to even to be detected by Anti-Virus programs.
Please suggest a software program designed to detect spyware and Trojan viruses. So that we can follow you simply.
@christmas flowers
Malware bytes antimalware is very useful in detection and removal of spywares and trojans this is the link http://www.malwarebytes.org/mbam.php and it is free.If you want to go for paid versions Kaspersky Internet Security or Macafee or Norton Internet Security suits are better.
Agree with Venkat, malwarebytes anti-wares is indeed a good one and its free. I would suggest you to try out the Bitdefender Internet version, paid version.
Thanks
David Cheong.
PM to Venkat,
Thanks for information. I expected it from Amit, but you have given the answer. I think Amit is busy to answer the comments.