I have three computers connected to the same network. Every so often, on the
laptop, I get the error message "Internet Explorer cannot display the
webpage ….." and a similar one if using Mozilla.

It’s still got the internet connection – emails are coming in. But I have to
shut it down and reboot and then it’s fine for half an hour or so and then
does it again.

I’ve tried deleting browsing history and files and cookies, I’ve tried
resetting the modem and I’ve disconnected the firewall.

OS is XP, The router is Netgear RangeMax