It sounds like you are doing a discovery and automatic agent push\roll-out. So, depending on the situation, you may have what Erik mentioned, the admin share doesn't exist (which is typically caused by simple file sharing being enabled, or a GPO).
If the machines don't exist, the discovery still found the machines, and they are most likely listed in the All Windows NT\2000\XP\Vista\2003.... with NO Altiris Agent. If enabled with the default settings, the automatic agent roll-out will keep trying to push the install to machines in that collection.
So, are you doing a discovery and\or automatic agent push?
or
Is simple file sharing enabled?