Last night I ran a virus scan and it showed a Trojan Horse virus
These is no such thing as a "Trojan Horse virus." Trojan Horse and virus are two different things. A Trojan Horse tricks you into installing and authenticating it. You install something with your password, and it comes with a nice trick inside. A virus -- and there are no viruses for Mac "in the wild" (that translated means no Mac viruses) can get in behind your back.
If it is a Trojan Horse, one way or the other you had to have installed it. You could run the free, demo version of MacScan (be sure to update after installing.)
http://macscan.securemac.com/
ClamX will scan only for known Windows viruses, which will not affect a Mac. That it didn't report anything doesn't mean much.
In addition, Norton has a terrible reputation for wrecking OSX. It is not recommended you use it. Many who have will tell you to uninstall it ASAP.