A possible cause is that your PC's are not all on the same network and
the router interface is on the same network as only the one that has
Internet access?

Have you checked the IP address, subnet mask and default gateway of
all the PC's? Start by taking the details of the PC that has internet
access and then make sure that the sub-net mask and default gateway of
all other PC's is the same as that. The IP addresses should differ by
say one in the last number only and that number should be between 1
and 254 (!).

If you don't know what I am talking about, post again and I'll give
more details.
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