Did you try rebooting in safe mode to delete?
Also you may be able to block the trojan from starting at boot by disallowing, or deleting it from the windows startup. There's a nifty little free download program called Startup inspector that will list all the items in startup, give you information on what each one is, and allow you to disable, or delete it.
Or...if you're using XP, doesn't it come with a task monitor thing that will allow you do see what's running and kill the process. If not, I'm pretty sure you can still download free task monitors. The one I use is old. It's not available anymore, so I can't think of one to recommend of the top of my head.
Sometimes malware comes with stuff like registry entries that will reinstall the thing after you clean it on the next reboot. That's why I suggest googling the name of the trojan. It's tricky in the registry if you don't know what you're doing.
BTW, you've backed everything up you can't afford to lose, just in case something goes wrong, right?
Often other people have had the same problem you're having, and you'll find an expert on some website explaining in detail the proper way to clean the same malware you're having the problem with. Again, that's why I say Google it first.
The problem with using an anti virus such as Norton (other than the well known fact Norton is buggy) to clean a trojan is they often specialize in viruses, and neglect some trojans expecially commercial, spyware trojans.
Edit: You were talking about the worry you might delete something you need. One thing I've heard of people doing is rather than delete it, rename the extension. Eg. If you have something called something like malware.dll, rename it malware.old. If things screw up go back and give it it's old extension. If it fixes the problem with no ill effects after a couple of days, delete it. I'm not telling you to do this, I'm just saying I've seen it done.
Even better advice is Google the name of anything, if you don't know exactly what it is.
