I did an unattended install of the Citrix WebInterface on a testmachine and it failed with error 258.

The logfile clearly indicates the reason:

[ ERROR ] com.citrix.wi.install.SetupErrorReporter: Error 258 occurred: ASP.NET 2.0 must be registered and enabled in Microsoft Internet Information Services before the Web Interface can be installed.
We can register ASP.NET in IIS with the Aspnet_regiis.exe tool that comes with the .NET framework.

The commandline would be:

aspnet_regiis.exe -enable -ir
We do need to make sure we are using the proper version of the tool (x86 or x64), I did that in the Embedded Altiris Script:
batch Download
rem Register ASP.NET in IIS

@echo off
if %PROCESSOR_ARCHITECTURE%==AMD64 (
	Set InstallPath=C:\Windows\Microsoft.NET\Framework64\v2.0.50727
) else (
	Set InstallPath=C:\Windows\Microsoft.NET\Framework\v2.0.50727
)

"%InstallPath%\aspnet_regiis.exe" -enable -ir

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