connect PC to a switch port on router.
unplug router's power
if possible, set pc's network card to half duplex, 10mbit
set pc's ip to 192.168.1.10
run tftp-hpa:
# tftp
(to) 192.168.1.1
tftp> trace
Packet tracing on.
tftp> rexmt 1
tftp> timeout 60
tftp> binary
tftp> put
(file) imagenamehere

now power up the router and wait