My up-to-date Win7 (32 bit) starts refusing to connect to servers on the WAN side randomly. There are other PCs in our network using the same Internet connection and they're ok (an ADSL router). I also have Ubuntu installed on the same PC and it's ok too. So the problem is clearly in this particular Windows installation. When this happens I have to disconnect from the network using the control panel item and then connect it again. Nevertheless, the LAN connection itself is always ok, i.e. I can see the ADSL router settings, share files with other network users, etc. It looks like it just won't pass traffic to the WAN side. There is nothing special about my PC and the only two programs that have something to do with networking are MSSE and a web browser (of course there is also Windows Firewall but its settings are not changed). Any diagnostic tools, advices? Anything is highly welcomed. Thanks a lot!

Make sure that the drivers of the NIC are updated on the machine. Also when it is not able to connect to the internet, are you able toping the DG?