Your problem is SO easy to fix, i dont understand why no-one has mentioned anything about it already.
The computer with the new motherboard has created a new connection to the internet. This connection is auto-default set to "Obtain IP Address Automatically" and since you are not using a router or other DHCP Server (one that assigns IP addresses when requested) your computer is using the autoconfig IP address (the 169.* one).
To fix this, go to the "host" computer, and find the IP address it is using (cmd; ipconfig /all etc) then set your computer's IP address to this number + 1. For example, if the host's is 192.168.1.10 set your computer to 192.168.1.11. The subnet mask should be the same as the host computer and the default gateway and DNS should be the host computers IP address.
Hope that cleared things up
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