Ping/Networking general difficulties

Hi,
Hopefully someone can help. I am running an iMac with Mac OS 10.3.7 and also a PC with Windows 2000. Both are connected to a Netgear Firewall/router. For some reason I can no longer see the iMac from the Windows machine OR the Windows machine from the iMac. I can't even ping either of them. (Both can ping the Netgear though). And both machines can use the internet just fine.
Netgear: 192.168.0.1
iMac: 192.168.0.2
Windows: 192.168.0.3

What could be wrong to not allow the two computers to see each other? (I'd like to be able to share files between the 2 machines).

Thanks!!!!!!

Turn on Filesharing on both machines.

Unfortunately that had no effect. I still can not ping either machine (from either machine).
Any other ideas?
Thanks.