When DHCP assigns an ip address, it usually also gives the IP of the default route.
For example your IP might be 192.168.18.12 and the default route might be 192.168.18.1
route -n will show you what your current routes are
Code:
Destination Gateway Genmask Flags Metric Ref Use Iface
x.x.144.0 0.0.0.0 255.255.252.0 U 0 0 0 eth0
0.0.0.0 x.x.144.1 0.0.0.0 UG 0 0 0 eth0
The second line is the default route. Anything I don't know how to route will be sent here. If thi line is missing and you are trying to ping an address that is not included in any of the subnets, the destination will be "unreachable".
If you can find the default route that is assigned to the Windows PCs, you can add it using
route add default gw 1.2.3.4
or add it to the "static" section in /etc/network/interfaces
Who is assigning the ip addresses in your setup? Your linux box, one of the Win PCs or someone else?