Rans, try plugging you XP computer network socket straight into the powerline extender socket ie don't even use the MR314. That would show you if it was the MR314 or the XP computer. If it's the MR314 that is slow, you can buy cheap
network extenders (not routers, per se) that plug into the powerline adapter Ethernet socket. I have one and it works well: full speed wifi at the other end of the house.