If it is a trojan, the best thing to do is reformat. Back up the dats you want/need and reformat!!! (It is sometimes not a bad idea to reformat once a year anyway - not that I always do that but on occasion we do).
There are programs that will ID trojans - like Spybot and Ewido Anti-Malware. HijackThis. is also a good program for getting rid of trojans and malware that have attached themselves to your registry if you do not want to go to the extreme of a reformat. Trojans are insidious and you should get rid of it before even using your computer - do you know what trojan it is?
"There are two means of refuge from the miseries of life: music and cats" Albert Schweitzer