Ok this maybe a long shot but can you swap your DNS server order. At the moment it seems you are going to 194.168.8.100 first. Can you make sure that 194.168.4.100 is the primary DNS server.
When I perform an nslookup against each server for dell.co.uk they provide the same results but list the IP addresses in different orders.
I'm sure we can resolve this
Do you have any
Static Content Filtering configured on your router. Also with your router you don't need any software firewall installed on the PC unless you are crazy paranoid, uninstall it for the purpose of testing :-)