If AV definition downloads are failing on the core via a manual or scheduled task, renaming the Win and Mac folders in C:\Program Files\LANDesk\ManagementSuite\ldlogon\antivirus8 will force the definition configuration to reset and recreate the configuration files repairing the download and fixing the issue.
Remote Console Mapped Drive Issue
In some cases running a manual AV Definition download on a remote console can cause a "Z" drive to be mapped to the remote computer and the AV Definitions to be sent to the remote computer then when complete, be transfered back to the core. This was verified by a client using wireshark during a remote console AV update. The AV definition download will usually complete but it will take much longer having to transfer back and forth twice across the network and other issues could arise that might cause it to fail. It is best to run the AV download from the core server itself and not a remote console.