A trojan horse virus is something that installs itself on your computer, usually from a webpage you've visited or by downloading and running a program that has been infected.
Because of firewalls and NAT routers, programs on the internet can't get into your computer. However, your computer can make connections out through firewalls and NAT routers, which is what your web browser does.
Trojan horse viruses are bad because they connect from your computer like a browser to somewhere on the internet to do bad things, like upload your webpage passwords file (like for hotmail, yahoo, banking sites) or use your computer to send viruses around.
What to do: First unplug your computer from the internet. Next, go on to another computer and download some anti-virus software from a reputable place. Here's one:
http://free.avg.com Create a CD with the downloaded program on it. This is important as the virus may try to infect the anti-virus program too. Run the anti-virus program from the CD and hopefully it will kill the virus.
There's more to it than that as ideally you should boot the PC up from a bootable CD so that the virus program is not started automatically when the PC starts up from the hard drive but that might be enough up there.