If you look in the Altiris server log you might see inventory records being discarded, I've seen quite a few for the format of the date and time of the virus signatures not being in the correct dformat for a SQL date/time field. I suspect that this is why it's turned off by default.
It's always been my understanding that Antivirus is one thing best managed by the dedicated console that comes with your antivirus prouct. Although people will often use Altiris to roll out the AV client to machines without it.