On the Desktop PC in network connections you should have two connections: "Local Area Connection" and something like "Speedtouch Connection" or something like "Connection <My ISP>". Leace the ISP connection alone and work on the local Area Connection only and set that ones IP to 192.168.0.1 and it's DNS servers to your DNS servers which you can discover by doing ipconfig /all.

You also need to share the speedtouch connection or other PCs won't be able to make use of it.