Latest versions of Norton detect the 64-bit ZCO as a virus and 'remove' it for you (preventing upgrade).
Basically, Symantec tracks who downloads what and if there isn't much data out there on people downloading a particular file, they automatically assume it's a viurs. Stupid, I know.
WS.Reputation.1 Technical Details | Symantec
Maybe someone from Zimbra should contact Symantec and let them know these files are OK.
The only way I found to actually install the file is to go through the Norton U/I to the quarnttine and then 'restore' the file and exclude it from future scans. Then I can run the installer.