Can't ping local computers

[tron_thomas]

I have a small network consisting of two computers.

One computer runs Mac OS X. The other computer dual boots between Windows XP Professional and Fedora Core 2.

When I'm running Windows both computers are able to ping each other. When I'm running Linux, neither computer can ping the other.

What could be the cause for this?

[Bana]

Is the network card being initilized? What is the output (as root) of /sbin/ifconfig

[tron_thomas]

Yes the card is initialized.

Both computers can access the Internet via a router connected to a DSL modem.
Both computer can ping the router. They cannot ping each other.