Installation problem NIS 2013

While I managed to get it installed on my full-size laptop, it refuses to start the installation process, on my tower.  The particular one I have, can be installed on 3 PC's.  Instead of just one.  While I will check out the possibility that, a family member installed it on two PC's but didn't tell me, I would like some ideas' as to what might be giving me the red 'X' at the product key.  I have looked at the sequence of the numbers and letters, probably twenty times.  I still can't figure it out.

 

NIS 2013 Requirements-

 

OS:

 

Microsoft® Windows® XP (32-bit) Home/Professional/Tablet PC/Media Center (32-bit) with Service Pack 2 or later

 

Microsoft Windows Vista® (32-bit and 64-bit) Starter/Home Basic/Home Premium/Business/Ultimate with Service Pack 1 or later

 

Microsoft Windows 7 (32-bit and 64-bit) Starter/Home Basic/Home Premium/Professional/Ultimate

 

Microsoft Windows 8 and Windows 8 Pro (32-bit and 64-bit)1

 

Hardware:

 

300 MHz for Microsoft Windows XP, 1 GHz for Microsoft Windows Vista/Windows 7/Windows 81

 

256 MB of RAM

 

300 MB of available hard disk space

 

CD-ROM or DVD drive (if not installing via electronic download)

 

My System-

OS: Windows 7 Professional

CPU: Pentium R 2.9Ghz

RAM: 4GB

Avail. Space: 300GB(500GB HD)

CD/DVD: Combo 

 

So I am stumped, why it won't allow me to install on my tower.