I also use BT/Yahoo and they really do catch 99% of viruses before they arrive in your inbox. However, the odd one still gets through so it's still necessary to use a good antivirus software. But the trojan you have picked up is more likely to have come from a website you visited, and not neccessarily a dodgy looking one. Hopefully, the advice Simon has given you will help you to restore your computer to its former state. If it's persistent, don't hesitate to come back.