Are you running the installer from an administrative account?
Even if you are, I'd try right clicking on the installer and selecting
Run As Administrator.
Also see the tutorial section about setting up the true Administrator account and try from there: Built-in Administrator Account - Enable or Disable
If that doesn't work, the software may not be suitable for Windows 7. You may have to try Compatibility mode. Does the publisher of the software claim Windows 7 compatibility or is it some XP stuff from 10 years ago?