I would agree with mr apoc, either a virus or your firewall I would say. Maybe worth reinstalling TCPIP. If its XP find the most recent nettcpip.inf file and install it. If its vista you need to run netsh int ipv4 uninstall, reboot, netsh int ivp4 install, reboot.
May also be worth checking your bios for anything related to lan / wake on lan. I guess it's possible it was trying to go into a halt state when lan was lost.
Just ate 5 Cadbury's Double Deckers in 30 minutes