After installing Vista SP2 on x64 and x86 machines I noticed that on reboot the systems do not have a connection to the internet.  The local lan is available.  If I run an IPconfig on the machines the "gateway" is missing.  I can add this information manually and it will work, but upon reboot it disappears.   All was well until reboot.  Once rebooted gateway is missing. Just click on the profile and it works.  

 sometimes by no reason when wakeup vista will happen "gateway" missing.

1. Open the registry with regedit.exe

2. Go to the path: HKLM/System/CurrentControlSet/Services/Tcpip/Parameters/Interfaces/

3. Here you'll need to select the CLSID of your networkadapter where you change the settings. You'll probably recognize it by looking at the value IPAddress that will have the IP of this adapter.

4. Open the value "DefaultGateway" by doubleclicking it. You will see a list of all the gateways that disappeared! (maybe in your case it will be only one, in my case there were due to lots of testing several addresses) Very likely, the first line will be empty. I'm not sure, how this little flaw can make such a big problem. However, if you manually remove this first empty line, click OK and reboot your system everything should be working fine again.

After doing this, everything seemed normal to me. I could change the GW manually . After a reboot everything was still there. Of course it needs to be verified in a long-term use if it fixes it forever or if the problem comes back at some point.