Trying to understand "Licensed Installs"

I get that I'm aloud to install on three computers.  How is this tracked and how can I view what computers (of mine) are identified on this installed list.  Also another post stated that I can re-install as many times as needed as long as it's on the same computer ?  I reformatted and reloaded windows (which I like to do periodically) and now I'm getting the message that I need to buy another key because I have no more available installs.  If I have an old blow up computer identified by norton as having a install I need to blow that away.  How do I clean the slate and start over ?

Please help Nathan has not been that helpful.

 

Thanks in advance

Sha Lung