The term "Trojan Horse" is a generic term used to describe a virus which pretends to be something which it is not i.e., a legitimate program, email etc, hence the "Trojan horse" moniker ('cos the virus bit is hiding inside).
The virus you have been infected with will have a specific name, and if you can identify that, we can point you in the right direction.
Scan your computer with an anti virus program (AVG is good and free) and see what happens.
Get AVG here:
http://free.grisoft.com/doc/2/