The only built-in way to manually disconnect and reconnect in Windows 7 that I've been able to find, requires a lot of mouse clicks and even on my Core-i7 steel nerves when it starts "troubleshooting" to reconnect very slowly.
I found a better way and because lots of people in several forums have been asking for it I hope it's going to be a Sticky.
Download http://itsamples.com/downloads/network-activity-indicator.zip (always scan downloads for viruses)
Activate it, it will show up as the Windows XP network activity monitor in the tray.
Right click it and click Network Connections.
You've now opened an explorer Control Panel > All Control Panel Items > Network Connections.
(I'm sorry, I found no other decent way to get to here)
Lets say you want a connect/disconnect Local Area Connection toggle on your desktop. You simply right-drag Local Area Connection from this control panel to your desktop, once you release your mouse button click on Create Shortcut. You can create such toggle shortcuts for every network connection that shows up in the list.. A simple double-click on the freshly made desktop item connects or disconnects whenever you like.
If you don't like the XP style network activity indicator, feel free to remove it now. But remember, once you close the Network Connections window, there is no decent way in Windows 7 to reopen it. As far as I know only the Network Activity Monitor knows how to do that. By the way, I do like the visual indicator of network activity.
Why is Microsoft constantly overpaying nerds for shaving off perfectly healthy logical features from their operating systems?