Can the Windows machines see the Ubuntu machine ?
Which version(s) of Windows ?
Can the Ubuntu machine see the Windows machine ?
What workgroup are the Windows machines on ?
is samba installed in Ubuntu ?
What workgroup is the Ubuntu box set to (or have you not changed it at all) ?
Do you use static (local)n IP addresses ? .. or do your PC's get their IP's assigned by the routers DHCP server ?
If you don't know the answer to any of these questions .. just say so, and we'll be able to point you in the right direction