This problem occurs when you got a PPPoE connection on Windows Vista.
Here's the guide to fix the problem:
Go into the Control Panel
Double click on “Network and Sharing Center”
Click “manage network connections” on the left.
Right click on “Local Area Connection” and choose “properties”
Click the “Configure” button
Click the “Advanced” tab at the top
Under “Property” choose “Priority and VLAN” or “VLAN and Priority” (It varies for different cards.)
Under “Value” select “Disabled”
Cause: One of the updates from Windows.