When you say manually are you speaking of installing VirusScan locally from the desktop of the server or are you trying to do this from an install on the network in a share?
I just mean running a client task product deployment from ePO manually instead of waiting for it to be done automaticaly from on the tags I had set up to trigger automatic installation..
For the scheduled task that's failing, check on the client machine itself: check in the agent log that the task is being invoked correctly. If not it means the client machine may not have received the task.
If it's being invoked but failing, check the mcscript log - there should be details of what's failing in there. Feel free to attach the files if you would like us to take a look.
If you were to RDP to that machine and perform the installation manually do you get an error prompt that doesn't get relayed to ePO? I had a similar issue that because there was a UNC in the environment path would not install VSE 8.8, but there was no indication in ePO. I saw the issue on the local machine which allowed be to setup a work around in ePO to work through this issue on the few machines with this setup.