PLEASE HELP! Trojan has corrupted my computer!?

Turn your computer on, then turn it off at the mains or via the switch on the power supply at the back. when you have done this turn it back on and during start up ( black screen, white text) you will be presented with some options. choose "Safe Mode"... then let it start up ( it will look really different and it will say loading files with scrolling text before asking you to login. then run your antivirus and remove all viruses.... reboot your pc and it should be fixed !