let winpe boot from usb

[1] using virtualbox 4.1.2
[2] make a usb raw disk vmdk --> usb.vmdk [the key is \\.\PhysicalDriveN]
    VBoxManage internalcommands createrawvmdk -filename usb.vmdk -rawdisk \\.\PhysicalDriveN
[3] make an new VM
[C] usb.vmdk
[cdrom 1] winxpsp2_pe
[cdrom 2] winxpsp2
[4] boot from winxp_sp2_pe
[5] format c:
[6] boot from winxpsp2
[7] recover --> fixmbr fixboot
[8] boot from winxpsp2_pe
[9] cd winxpsp2\i386\
[10] winnt32 /cmdcons
[11] copy /e winxpsp2_pe\i386\*.* c:\minint\
[12] c: cd \
[13] copy cmldr ntldr
[14] copy minint\winbom.ini \

[15] your usbstick had winpe bootable ability