What is the trojan virus backdoor-bac.dll virus, and how do you remove it?

Wikipedia has a good definition here:
http://en.wikipedia.org/wiki/Trojan_horse_(computing)

In essence, a Trojan relies on you to do the infection, by downloading something that looks interesting or useful to you, but contains a hidden function that you would probably not agree to. By this simple definition, one could argue that a program with spyware attached is a form of Trojan. This would be an accurate description - if the software company did not alert you to the inclusion of the spyware in its EULA.

Just to clarify - it's just Trojan, not Trojan virus. There's nothing virus-like about a Trojan because a virus gets around under its own steam.

What you get when you don't use one.

Because there are multiple variants of this backdoor and multiple components associated with it, there is not a simple one line answer to this question. Following are some links to removal instructions but having somebody experienced with computers look at it would be the best solution. Second best solution is to get a working up to date antivirus product installed, update the virus definition files, boot to safe mode and run a full system scan. I like to run a tool such as "hijackthis" to get a look at all load points and take in the environment to manually clean things up first and then run an antivirus tool to find anything I may have missed in compressed files somewhere.

http://securityresponse.symantec.com/avcenter/venc/data/backdoor.mutny.html
http://vil.nai.com/vil/content/v_101152.htm

"In the context of computing and software, a Trojan horse, or simply trojan, is a piece of software which appears to perform a certain action but in fact performs another such as a computer virus. Contrary to popular belief, this action, usually encoded in a hidden payload, may or may not be actually malicious, but Trojan horses are notorious today for their use in the installation of backdoor programs. Simply put, a Trojan horse is not a computer virus in most cases. Unlike such malware, it does not propagate by self-replication but relies heavily on the exploitation of an end-user (see Social engineering). It is instead a categorical attribute which can encompass many different forms of codes. Therefore, a computer worm or virus may be a Trojan horse. The term is derived from the classical story of the Trojan Horse.

In the field of computer architecture, 'Trojan Horse' can also refer to security loopholes that allow kernel code to access anything for which it is not authorized."
Source and further information:
http://en.wikipedia.org/wiki/Trojan_virus