can't remove virus "backdoor.trojan"

A system scan with Norton 2006 reported a virus called "backdoor.trojan" and offered to remove it.  I hit "ok"  but the program was unable to remove the virus and stated that the virus required manual removal.  There was a link to the Norton site that suggested three steps: 1) turning off system restore, 2) updating virus definitions through Live Update or Intelligent Updater, and 3) re-scanning.  I did this procedure first by using Live Update and Live Update had one small section that failed to update; this re-scan again detected the virus and could not remove it.  I then updated the virus definitions through Intelligent Updater and this time scanned in Safe Mode and the scan did not detect any virus.  I immediately re-scanned in Normal Mode and again had the virus detected and the program could not remove it.  How can I get the virus off of my computer?  Why is the virus not detected in safe mode, but is detected in normal mode?  Please help! Thanks...