After searching around on the Internet for a week and having some puzzling results I'm posting my problem here.
It arrive mid last week when one of my computers had a connection problem, it was connecting to my router (Motorolla surfboard SVG1501) on wifi but only on Local.
First I thought it was just the line that had gone down, but my phone (Galaxy SII) was connected to the same Wifi signal and I had Internet access...
I started with the simple things (computer restart, modem reset, ipconfig, netsh, connecting the PC to the modem via cable, ...) and ended up making a restore of the computer to an earlier stage when I kenw Internet was working - nothing did it.
As I have a switch (TP-LINK TL-SG3210) connected to the modem I removed it from the network and it after restarting the wifi board on the PC I had Internet again...
Then I made a test and as soon as I plug the switch the Internet connection is gone.
Now the Network is/was working for more than two months and never had a problem before, so I suspect some "Security update" to have caused the problem (my Android Phone had no problem to get Internet access).
For info this computer is running W7 and I have another one running Vista (this one wired, not on wifi) and both had the same issue!
So if anyone has a bright idea I'm more than happy to test it (I'm rewiring the complete network to see if this changes anything).
Since that’s a managed switch, it might have its own DHCP server. And since your Motorola Surfboard SVG1501 also has a DHCP server, that could create a conflict. In some cases, your clients may be getting a response from the switch's DHCP server instead of the router's DHCP server, and so they end up misconfigured. The solution (if I'm right) is to disable the DHCP server on the switch.
Thanks for your reply and suggestion, but yesterday evening I was rewiring the whole network and also remembered o check the switch configuration!
It seems that following an electric black out last week (I was not aware of) the switch had changed the configuration back to the defaut and it's IP address was no longer DHCP but static and the same as the modem.
Changed that and all got back to normal!
Dumb me not checking this before but at the same time had no reason to think the configuration of the switch had changed. I was already cusring Microsoft thinking it was coming from one of their epic "Security updates" - my bad.