i know it's an old tread but it might bu usefull for someone else, to do that you will need an other pc connected to te same network as your other old-PC, so on your PC wich you have windows install on ( i presume ) use Tftpd32 it's a dhcp and network booting daemon.
Set tftpd32 to have grldr as boot file.
Download plop boot manager
on the old-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 grep, select command line
type in :
map --mem (pd)/plpbt.img (fd0)
then you'll be in plop boot manager and you'll be able to boot from usb from now
this is the only way i've found for my old fujitsu laptop. doesn't support usb booting, neither have a floppy/cd-rom