NIS2009: Trojan horse suddenly detected

Almost a month ago I switched to NIS2009, coming from NIS2008.

During the last month a full virusscan was performed at least each week.

Now suddenly a trojan horse is detected in a zipped file which is on my system since december 2007.

 

This is not the first time I observe this kind of strange behaviour.

The risk is called "Trojan Horse" and looking at the details it says that this was discovered 19 feb 2004 (http://securityresponse.symantec.com/security_response/writeup.jsp?docid=2004-021914-2822-99)

 

I don't understand why this trojan was not discovered by NIS2008 and the last weeks also not discovered by NIS2009.

Or is it a new type of trojan which was discovered the last week. But why I don't see that looking at the detail-information?