Right now, i've just tried building out a Windows XP machine manually, using F6 to specify the AHCI Drivers, which has completed successfully. But when i try to take 'Create Disk Image' of this, Altiris reboots machine with 'Restarting Computer to prepare for imaging..' message, then it craps out with the error 'No Boot Device available' as per my earlier post.
So, it looks to me that problem isn't with the OS Image I'm trying to use, its getting Altiris to communicate with the Image using the Drivers loaded - or failing to load - in WinPE.