Hey changintimes, welcome to Bleeping Computer
You are right in saying there is a legit ctfmon.exe on XP.
How do you know that this file is a trojan?
Are you able to tell me where the file is located on your PC?
Try a scan of your computer and see how many times ctfmon.exe is found.
Let me know the path-names.
Also, have you run any anti-malware tools on this PC?
I'm assuming that your antivirus has flagged this file as being bad.
As a side note, don't try and edit the registry for the time being.
Even though it wouldn't be catastrofic, you don't want to delete the legit one.
If this file is a baddie, we can easily remove it without touching the registry.
It's a dangerous place, and you should always back it up before editing it.