I'd assume your using an upgrade media for the install and not a retail version. Should be an easy fix with the following:
Open regedit.exe with Start Menu Search and navigate to:
Change MediaBootInstall from "1" to "0".
Start Menu on the Command Line utility. Right-click this shortcut and choose "Run as administrator."
Click Yes to the UAC prompt.
In the command line window, type: slmgr /rearm press enter
wait for the "Command completed successfully" dialog.
Then, close the command line window and reboot.
After Windows 7 reboots, Right Click Computer select Properties select change product Key.