Having trouble remove a Trojan Horse

I have a computer that has a trojan horse on it, named ‘Trojan Horse’. I have symantec corporate edition, and I have found one trojan. It is currently residing in the pwdmon.dll file in \windows\system32, I have been to the symantec site and they told me to run a scan then just click remove, unfortunately it is rarely that simple. I also followed the directions and turned off system restor booted into safe mode, scanned the file and found the virus again; then I tired to delete it and it was a no go. Anyone have any ideas of how to remove this? Thanks in advanced