Retail 'Key' problem

I bought "Office Home & Student" retail. On the back of the box it says
"licensed for 3 PC's". I have 2 computers. I successfully installed this on
my first computer
and the product key was accepted. On the 2nd computer installation it
keeps refusing the same product key so I cancelled the installation. Why
doesn't it accept the key for the 2nd computer? What do I do?
--
MC