Just like Mr. Mixx says. He's most likely right.
I think that you have the "Allow the computer to turn off this device to save power" turned on. Now im not sure. But i think if you turn this off you should not get that warning.
To turn this off:
go to "My network places".
Then click "View Network connections".
You should see "Internet connections" in the list.
Right click on the icon that you see> Properties
You are now in a window that says "Local Area Connection Properties".
On the first tab (General Tab) You will see a box that below that, that says "Connect using" Then below that your network card.
Next to this is a configure button. Press configure.
It will open a new window called "Youronboardlanset Network connection properties"
Click on the "power management" tab.
Now you see the check that says "Allow the computer to turn off this device to save power". Uncheck that.
Now you just need to let your comp go into standby and see what happens.
Let me know.
Thanks,
-Reaper.