Duplicate GUID causing problems in RIS

I have a bunch of computers where they messed up the GUID in the MFG and
made the GUID all 0s. Now when I try to install WinXP on them via RIS,
everyone gives me a error after the first about duplicate GUIDs. I also
cannot bring up more than one at a time because they could get the same
computername.

Before computers had the GUIDs, RIS would use the MAC padded with 0s. I
know that when RIS is being used to install windows it is pulling the GUID
from the computers eviromental variables along with other variable that also
get set via some of the questions on the RIS screens.

I have managed to edit some of my RIS screens in the past to do things like
allow users to enter the Windows KeyCode on a RIS screen so I could pass the
variable to the SIF file and prevent RIS installs from stopping at the
keycode screen. But is there a way to override the GUID variable during the
RIS setup?

Here is what I was thinking. I would have one of the pages on the RIS
install setup so it shows the GUID and the MAC padded with twenty 0s. If
the GUID was messed up, I could select the MAC with padded 0s and that would
get passed to AD. I have attempted to do this by adding a line

SET GUID="00000000000000000000%MAC%"

But that did not seem to change the GUID variable.