What is a Trojan horse?
Trojan horse attacks pose one of the most serious threats to computer security. If you were referred here, you may have not only been attacked but may also be attacking others unknowingly. In today's computer world, a Trojan horse is defined as a "malicious, security-breaking program that is disguised as something benign". For example, you download what appears to be a movie or music file, but when you click on it, you unleash a dangerous program that erases your disk, sends your credit card numbers and passwords to a stranger, or lets that stranger hijack your computer to commit illegal denial of service attacks like those that have virtually crippled the DALnet IRC network for months on end.
Trojans can be spread in the guise of literally ANYTHING people find desirable, such as a free game, movie, song, etc. Victims typically downloaded the trojan from a WWW or FTP archive, got it via peer-to-peer file exchange using IRC/instant messaging/Kazaa etc., or just carelessly opened some email attachment. Trojans usually do their damage silently. The first sign of trouble is often when others tell you that you are attacking them or trying to infect them!
How do I get rid of trojans?!?
Here are your many options, none of them are perfect. I strongly suggest you read through all of them before rushing out and trying to run some program blindly. Remember - that's how you got in this trouble in the first place. Good luck!
Clean Re-installation: Although arduous, this will always be the only sure way to eradicate a trojan or virus. Back up your entire hard disk, reformat the disk, re-install the operating system and all your applications from original CDs, and finally, if you're certain they are not infected, restore your user files from the backup. If you are not up to the task, you can pay for a professional repair service to do it.
Anti-Virus Software: Some of these can handle most of the well known trojans, but none are perfect, no matter what their advertising claims. You absolutely MUST make sure you have the very latest update files for your programs, or else they will miss the latest trojans. Compared to traditional viruses, today's trojans evolve much quicker and come in many seemingly innocuous forms, so anti-virus software is always going to be playing catch up. Also, if they fail to find every trojan, anti-virus software can give you a false sense of security, such that you go about your business not realizing that you are still dangerously compromised.
Anti-Trojan Programs: These programs are the most effective against trojan horse attacks, because they specialize in trojans instead of general viruses. A popular choice is The Cleaner, $30 commercial software with a 30 day free trial. To use it effectively, you must follow hackfix.org's configuration suggestions [ext. link]. When you are done, make sure you've updated Windows with all security patches [ext. link], then change all your passwords because they may have been seen by every "hacker" in the world.
Here is a good link for a virus scan program http://www.download.com/AVG-Anti-Virus-Free-Edition/3000-2239_4-10320142.html?tag=mncol
Install it, update it and run the scan.