Try this on both computers
start menu,
run,
type cmd
press enter
type ipconfig /all >c:\ip.txt
press enter
open my computer
open C drive
open ip.txt file
Copy the contents of each text file and paste here one at a time

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Have you tried swapping the cables connecting the PCs to the router? Take the cabe from the working PC and plug it into the PC that isn't at the moment and see if anything happens.