I'm running XP.
I've restored the factory settings on the PS3 and restarted and powered down the router, then started from scratch but still the same problem.
I contacted my ISP again, they talked me through fiddling with the settings on my PC (Control Panel > Internet Options etc.) but all was as it should be. He suggested something on my PC (like an Anti-virus software) was blocking the PS3 for connecting to the net. He didn't see any problems with my internet connection.
So I disabled my Anti-Virus (Avast) and firewall softwares for a while to see if they were the cause of the problem. But the network still disconnects when the PS3 is plugged in.
It's getting really frustrating at this stage

. I've tried everything possible it seems and yet nobody knows what the problem is.