2 computers 1 product key

Greetings,

 

I currently have Internet Security 2011 on my desk top.  I want to install said subscription on my lap top.  After submitting the product key it tells me that I have exceeded the number of allowable installations.  There has been trial subscriptions in the past.

 

I though that I can add a second computer without buying a second key. 

 

 

Help!

 

Any responses appreciated.  Thanks

 

jb