Network connection problem

I have a bit of a problem with connecting my two PC's via my hub. One minute they are connected fine then next then loose connection with the exclamation mark showing with the message "Limited or no connection". When I unplug the hub and connect one PC to the modem it sometimes gets connection to the net but then loses it with the red X displaying with the message "Network cable unplugged". I have ran the network wizard on both PC's still no change. I am really baffled. I just about had enough time to send this post so really need some help here please.

You haven't got both PCs set to the same static IP address?
Either on the PC network card settings, or you've one set to static, which happens to be the same IP assigned to the other in the DHCP server of the router/hub?