Greetings!

If the Mac and Windows computers are on the same network, and ping fails on one but not the other, I do believe you will have identified an issue with the computer that is failing to ping. You may want to try disabling any firewall or internet security software, anything at all which might filter outbound traffic. If that does not help, you may try connecting the Mac to another network to see if the same occurs, as that may help trace the problem to your router instead of the computer itself.

Jarland