I just made a x64 build,
Tested with grub4dos ISO boot,
works ok here.
not tested virtually, not tested real-cd, not tested with 'Copy to USB' -->
@Aeolis
How you boot / test / prepare boot media ... ;)
virtually, real-cd, 'Copy to USB', ISO boot, .... ufd, usbdisk, sdcard, etc .....
this would help ChrisR to debug on which case failure might be hapening.
Tip: Use 'Main Configuration' 'Create Support Log' in
future 

@ChrisR
I feel using such thing
\Win7PESE\Finals\zz_Save_logfile.script
http://www.mediafire.com/download.php?jpfccibjupi0aikhopefully make old timers notice

(of course with a w7 themed picture

)
ps: notice only 2 zz, not 3 zzz, to make difference but also similarity , midway I can find

