Office 2k3 SP1 slipstream

[Matt]

Hello,

I was trying to slipstream SP1 into Office 2k3 using the directions at
http://www.petri.co.il/office_system_2003_sp_slipstreaming.htm , but it's
not working apparently because the office install is a package created using
an MST. So I get prompted for a valid key. I use the following syntax:

msiexec /p d:\sp1\MAINSP1ff.msp /a w:\PRO11.MSI /qb SHORTFILENAMES=true
PIDKEY=PidKeyHere

I get the following message: "A valid Product Key must be set to create an
Admin image"

Without the PIDKEY entry it gives a different message: "you have not entered
a valid product key"

Is there a work around for this?

Thanks in advance,
Matt