I have been infected with a trojan virus

first get a antivirus program. here are 2 free ones.
http://free.grisoft.com/freeweb.php/doc/2/

http://www.avast.com/eng/avast_4_home.html

You should only run one of the above antivirus programs. both are giving so you can make a choice.

the programs below can be run with each other.

then you need to get adaware and spybot s&d

http://www.download.com/Ad-Aware-SE...045910.html?part=dl-ad-aware&subj=dl&tag=top5

http://www.majorgeeks.com/download.php?det=2471

I am sure that one of the resident experts will pick up on this and give you more help.