My Visual studios registration key works on one computer, but when I try to load it onto my laptop the key does not work

I purchased the vs2012 as a download from the microsoft website after running an eval version.  It works fine on this computer, but when I moved to my laptop and tried to convert the eval version there using the same key - it said it was not possible?

Am I allowed to run the program on both my computers?  If so, how can I get the version working on my laptop?

thanks

anton

Anton