How exactly do you know that you have a trojan? There are a lot of phony ads on websites that will tell you this to trick you into installing their software. My folks just received a message from Skype the other day suggesting the same thing.
If you already have an antivirus, say Norton for example, and something other than Norton tells you have a virus, then don't believe it.
You should do as saran suggested if you don't have an antvirus. I personally prefer Avast.