Think I have sorted it now, Thanks for all help.
This is what I ended up doing,
1.. Setting up each XP computer to a static IP address. eg, 192.168.0.199/255.255.255.0
2.. Setting up the DNS settings of my hardware firewall and DHCP server eg , 192.168.0.1
3... Gateway blank.
Un-setting all the 'Proxy' settings on the various programs.
In 'Internet options' --> connections settings --> lan --> un-set the proxy settings.
I can still get computer to computer file sharing .
I wanted to dis-able the internet on xp because I have stopped doing any updates to Anti-virus or malware or spyware and Windows updates, So did not want to let anything in via the back door.