Flashing green light of death

[Patrick1964]

I have a PC upstairs, and a laptop. When the PC stopped connecting to the internet (flashing green light of death on the Speedtouch modem) I was advised that there was a problem with the length of cable between the master socket ant the bedroom where the computer lives (appx 30ft). In light of the arrival of the laptop we decided to go wireless. Having finally got the modem / router installed at the master socket, it appears that there is no way that the signal is strong enough to reach the PC upstairs, so I decide to recable from the master socket to the PC room (hardwired not a crummy plug in extension lead). Fire the PC up, redo the connection wizzard to operate through the Speedtouch again, and all is fine. I shutdown the PC after removing ZoneAlarm (planning to install Avast) and on restarting that flashing green light of death is there again. WTF !!!! I'm in danger of seriously losing the plot here. Surely if it works one minute it should work the next ? Any ideas please, or the whole bloody lot is going in the river.