Set default values when customizing RIS screens

I have been using RIS for a while to install new Windows XP Pro SP2
computers as they come on the network. In order to speed up the process so
it does not stop in the middle of setup asking me for the Windows Key Code I
modified the custom.osc screen to prompt me for the Key Code when it is
asking for the Computer Name. Here is the customized section:

<FORM ACTION="OSCUST">
Computer name: <INPUT NAME="MACHINENAME" SIZE=63 MAXLENGTH=63><BR>
Directory service path: <INPUT NAME="MACHINEOU" MAXLENGTH=512><BR>
Windows Key Code: <INPUT NAME="WINKEYCODE" MAXLENGTH=29><BR>
</FORM>

You can see that I save the entered code to the variable WINKEYCODE. Then
in the RIS Image I modified the " ristndrd.sif " so that the variable
WINKEYCODE would be passed to setup.

[UserData]
ProductKey = %WINKEYCODE%
FullName = "Network Administrator"
OrgName = "%ORGNAME%"
ComputerName = %MACHINENAME%

I want to also customize other things but most of them will have default
values. How would I setup the default values, for example in this case if i
wanted FullName to default to "Network Administrator" unless I type in
something else?