Symantec reported Thursday that it has analyzed what it believes to be the <A HREF="http://zdnet.com.com/2100-1105_2-5221949.html?tag=zdnn.alert">first known threat to 64-bit Windows systems,</A> a virus labeled W64.Rugrat.3344. Company representatives stated the threat was merely a so-called proof-of-concept virus, a worm developed by someone to show that vulnerabilities are present in a particular type of system, and not a virus already spreading in the wild.