F8 not working in osd screen

Hello ,
i am deploying windows 7 with sccm 2007 sp2
i am booting from pxe and i see the screen of the SCCM OSD ( the sccm defualt wallpaper) but then the system reboots.
i tried to hit F8 but with no luck there is no responce from the system , i checked the WINPE settings and saw that the enable command ... is checked ...
can anyone thinks of why is crashes ?
thanks.

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Mkuriel