Run cmd.exe as admin!
On your Windows 7 operating system, open an elevated command prompt and type:
DISKPART LIST DISK SELECT DISK 1 (depends on your situation!) CLEAN CREATE PARTITION PRIMARY SELECT PARTITION 1 ACTIVE FORMAT FS=NTFS QUICK ASSIGN EXIT
Now that the USB stick is bootable, all you have to do is copy the WinPE source files on the stick and boot from it.
This approach also works for making bootable WinPE harddisk/USB Sticks, just add (G)ImageX.exe and you’re ready to make backups of complete systems (aka Ghost replacement)