|There's a couple ways to do this.|
The quickest/easiest way is if you have the "My Network Places" icon on your desktop. If you do, right click on it, select "Properties" from the menu appears. A window showing all network interfaces will appear and under "Status" it will tell you if a NIC is disabled or connected.
If the NIC you wish to use is disabled, right click on it and choose "Enable" from the menu.
It's worth noting, if you have a wireless interace that's enabled at the same time as a wired interface, they can often conflict and nothing works right. You definitely won't be able to have both enabled and connected to your LAN at the same time. So pick one or the other to use, and whichever one is enabled, disable the other.
If you don't have the "My Network Places" icon on your desktop you can get to "Network Connections" as follows:
Start >> Settings >> Network Connections
Or, if it's easier for you, you can go through:
Start >> Settings >> Control Panel >> Network Connections
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.