Back in my Windows days, when faced with this, the install/activation screen gave me a link where I could "challenge" the assertion and enter a series of codes. Once done, I believe it asked how many computers the software was installed on, and, eventually, it allowed me to activate the software. Kind of a pain, but from M$'s perspective, I guess it's understandable.
I have not faced this situation since switching to Mac/OS X in 2010, so I cannot say if this has any relevance to your situation or not.