After a restart of the Altiris Services or the Altiris Server some machines refuse to reconnect.
They are shown in the Computers Tree with the Inactive state icon:
The fastest way to resolve this is to restart the "Altiris Deployment Agent" service.
I wrote a little commandline tool to make this easy for myself, it's called AClientFix.
If you don't specify any parameters it will restart the services on the local machine. If you specify a Computername as parameter it will restart the services of a remote machine (admin rights needed of course).
