What OS are you trying to install?
To my knowledge, launching winnt32.exe should kick off an install, unless you are doing Vista. Then it is just setup.exe with the scripted install switches.
So boot to WinPE do the following manually:
1. Map a drive to your share with the OS software (winnt32.exe for 32 bit not sure what it is for 64 bit)
2. Run the installer manually
Does it work?
If not I guess I would find out from Microsoft if you can start the install of a 64 bit OS from WinPE 1.x. I though that I read that you could in WinPE 2.0 but we haven't gone to that yet...of course maybe I read wrong.
I have done a 64-bit scripted install of Vista on PE 1.1 before, so I would imagine any other 64-bit OS would work.
I was under the impression that the setup program for Vista-64 only comes as a 64 bit executable, which therefore won't run on a 32 bit WinPE. Am I mistaken?
here is a description for XP64/W2003 64: http://support.microsoft.com/?scid=kb%3Ben-us%3B925279&x=8&y=11
Additionally there exist a tool named ManualWindowsSetup.exe to install XP 64 from a x86 PE... I think it is a cracked XP 64 installer... (no existing download location found... http://www.msfn.org/board/install-windows-x64-bartpe-and-winpe-t70603.html)
Otherwise I've heard that some vendors (Dell, HP, etc.) do have got such a tool from MS to use one PE image to deploy several different hardware types, a winnt.exe to install 32 and 64 bit machines... (but I can't find the hint in internet any longer...:-( )
For Vista and newer the installation just lays down an ImageX image. This gives some interesting advantages. For one thing, you can lay down a 64-bit "image" from the 32-bit environment.
Here is some information for doing a cross-platform unattended install using Windows 7:
In short the solution is to have your unattend.xml file point to a .wim file from Windows 7 64 bit, even though you're running it using the 32-bit setup.exe. That means this functionality can be easily implemented in 8.8, we don't even need to wait until 9.0.
Vista should be the same, although I've only tested 7.