To do that you will need another PC connected to the same network as your other PC, so on your PC which you have Windows install on (I presume) use Tftpd32 it's a DHCP and network booting daemon.
Download Grub4dos.
Set tftpd32 to have grldr as boot file.
Download plop boot manager (google is your best bet).
On the client PC set it to a PXE-boot or network boot.
Turn off dhcpd from your router of you're using it.
Boot the old-pc.
Once your in grub, select command line.
Type in :
map --mem (pd)/plpbt.img (fd0)
map --hook
root (fd0)
chainloader +1
boot
Then you'll be in plop boot manager and you'll be able to boot from usb from now on.