Inserting the Key on a 2nd computer

HI,

 

I have 2 computers in my house, Both laptops.........

 

I had to install on my aunts pc Norton 2012 and it has 29 days left ( installed it last night ) 

 

this computer CAME with 2012 security and has 25 days left

 

I plan on buying a key once the time gets lower on This computer ( dining room lappy  the 25 days left) 

 

so if we can use the same key on 3 pc's then how do I use and insert the same key for the 2nd computer? ( my aunts PC ) 

 

Thanks,

Christopher