I'm wondering if it is a case that your PCs are not set to to obtain an IP address and DNS server automatically?
You don't say whch version of Windows you are using but for windows 7 you
Click on start, control panel, network and internet, network and sharing centre, left hand column change adapter settings, right-click on local area connection and click on Properties, in the new indow choose Internet protocol 4 from the list and click on properties, check to see if obtain an IP address automatically obtain dns server address automatically are selected. Click OK.
If you are using another version of windows you can google how to do the same thing.