Pretty tricky, the title says it all
cscript %systemdrive%inetpubadminiscriptsadsutil.vbs set w3svc/AspScriptErrorSentToBrowser true (to allow error message send to client)
On a x64 OS (I run Windows 7 RC x64)
cscript C:Windowswinsxsamd64_microsoft-windows-iis-legacyscripts_31bf3856ad364e35_6.1.7100.0_none_4b5800ce84aa413cadsutil.vbs set w3svc/AspScriptErrorSentToBrowser true