That sounds like it might be a bit serious (said this semi-dummy in reply.)
Couple of questions. How do you know the computer got a trojan horse? Did another security application or antivirus alert you to it? And if so, which one?
If not, what (if any) security applications are installed (antivirus, firewall, any suites etc)?
Any description you can provide in answer, especially if you can name the malware (malware= generic term for virus, trojan etc) may well help.
Also, do you run as an admin, or limited user account? (You may not know the answers. Just as you can.)
