For security, sometimes you do not want your computer to automatically start up allowing network connections. In Windows Vista there is a way to ensure that when your machine reboots, network connections will not be enabled. They will only be enabled once you log in the first time.
To do this, go to Control Panel -> Administrative Tools -> Services.
Then change the Network Connections service to Manual.
This will prevent Windows Vista from establishing or allowing any network connections until the first time you log in.