I have simple network with 2 PCs connected with
RTL8139 NICs. network was working fine for 1year.
Now all of sudden it is not working & one NIC shows
red light as soon as connected. I have reinstalled
drivers many times but there is no connection possible.
Can anybody help?

how are your network cables run? In wall, across the floor? Try swapping the cables and see if the problem (red light) shifts to the other computer. If not it might be possible the drivers or the NIC itself are bad. Also , if you are using a router/hub, try switching the ports that the bad NIC is connected to. I have had a couple hubs/switches with ports that went bad.