When the Apply OS step is run, it adds or overwrites a number of entries in the setup answer file.  The specific settings depends on whether the deployment is image-based (Apply operating system from a captured image) or setup-based (Apply operating system from an original installation source). 

In this post I'll indicate which values are implicitly set by the Apply OS step for Windows 2000, XP, and Server 2003.  In a subsequent post, I'll go through the same exercise for Windows Vista.

Note: there are a number of other values in the answer file which map to settings exposed by other task sequence steps (for example, Apply Windows Settings, Apply Network Settings, and Apply Device Drivers).  If a custom answer file already contains values for these settings, they are overwritten by the settings specified in the task sequence steps.

For a setup-based deployment, the values are assigned as follows:

[Data]
AutoPartition=1
MsDosInitiated=0
UnattendedInstall=yes

[Unattended]
UnattendMode=FullUnattended
OEMSkipEula=yes
OemPreinstall=yes
TargetPath=*

[GuiUnattended]
OEMSkipRegional=1
OEMSkipWelcome=1

[Networking]
InstallDefaultComponents=yes
Note: the InstallDefaultComponents setting may be modified by the Apply Network Settings step

For an image-based deployment, the values are assigned as follows:

[GuiUnattended]
OEMDuplicatorString="Microsoft SMS OS Deployment Agent"
OEMSkipRegional=1
OEMSkipWelcome=1

[Unattended]
OEMSkipEula=yes
UnattendMode=FullUnattended