I had a situation recently where I was unable to find my network ArcGIS dvd.  I only had the Single Use DVD which does not have the ability to select a network resource.  I made some registry changes and can now switch between Single an Network (Perfect for the road shows and when needing Info License in the office).

Notice how you are not able to select a network license:

To rectify, First open your registery via the Start -> Run and type in regedit.  This will bring up the registry.

Scroll down to License under the ESRI folder (see image below)

You will need to add a new registry string and modify 2 registry values.

1) Add a new registry string value by right clicking on the License Folder then selecting New ->String Value.  Type ArcGIS_Installed then right click on the new ArcGIS_Installed and select Modify.  Type TRUE into the value data text box.

2) Modify the value of LICENSE_SERVER and select your license server name (Don't forget to keep your @ sign in front of your server name -- See image below)

3) Modify the SEAT_PREFERENCE and change from Singel to Float

You can now change between Single and Network via the ArcGIS Desktop Administrator Panel:

Cheers,

Carlos