Q:

Windows 7 2nd system license

I bought Windows 7 last Oct. and installed it on my Mac laptop.   In January, I installed it on my Mac desktop.   It activated just fine, and I used it for a bit, installed some other programs, etc.   A couple of months later I went to use the desktop version again (I don't need Windows often on my desktop), and it tried to get updates, but MS decided it wasn't a legit version and now it looks like a "bootleg" (which it isn't).   I now understand that I need to pay for a 2nd license to get my desktop to be legal, and want to know what is the cheapest way to get the desktop fixed.   Is there an "upgrade" path?   What can I do? 

A:

Buy a license, then open Computer Management and scroll to the bottom.  There you will see "Change Product Key."  That will allow you to put in the new key and activate.  This will only work if they key you buy is for the same version that is installed. 
Alternatively - from the command prompt:
cscript slmgr.vbs -ipk xxxxx-xxxxx-xxxxx-xxxxx-xxxxx
cscript slmgr.vbs -ato

The first line inputs the key from the command prompt (you need the hypens) and the second line does the activation.