Hello and welcome to the form,
If you are switching computers and you are using a standalone license you will want to export the license from your computer and them you will import the license into the new computer after you have installed Civil 3D. See the article below
http://knowledge.autodesk.com/topics/installation-activation-licensing/manage-licensing/transfer-lic...
Once the license is transfered off the old computer you can download and install C3D on the new system. The download links are in your autodesk account at manage.autodesk.com or you can get them from the virtual agent using the method below.
Go to http://autodesk.creativevirtual15.com/autodesk/bot.html?isJSEnabled=1&businessArea=Root.Front%20Desk...
Select the 2nd Choice "Download Links "
Select your product
You are now presented with the choice to download your software.
Jeffry Asbury