backdoor.trojan is a group of viruses, it does not say which virus it is.
Norton usually gives you the name of the virus too... then you read up on internet, at northon, what the virus is about.
Then follow the instructions on how to remove it.
Usually its delete the infected files and remove references to the virus in registry, which is a bit tricky at times.
Then reboot and check if its gone.
Some Viruses you need to boot into save mode to get rid of, but mostly its enogh to delete the files.
If the infected file is importand you will have to find an uninfected version of it, usually cant restore it if its a program.
When the virus sits in an office document norton should be able to desinfect the document.
ciao schutt