Having the sticker doesn't actually prove that Windows 7 is genuine. The number on that sticker could have been used to register Windows 7 on a different machine and then the sticker put onto yours.
However, I'll assume that it is genuine. Microsoft's own solution isn't really one for the feint-hearted:
https://support.microsoft.com/en-us/kb/2008385
but the following might also work (and it's a great deal easier!):
Click the Start button.
Enter 'cmd' into the search box (without the quote marks).
You'll see 'cmd' listed under 'Programs'.
Right-click on it.
Click on 'Run as Administrator'
If you're asked for permission to proceed, grant it.
The window that opens will already have a line with 'C:\WINDOWS\system32>' in it.
Add 'SLMGR -REARM' to that line. (Don't include the quote marks. Don't precede that bit with a space. Do note the space in the middle of it though).
Press 'Enter'
You'll be asked to confirm your action.
Click on 'OK'.
Restart your PC and cross your fingers.
That will probably work. If not, try again, replacing 'SLMGR -REARM' with 'SLMGR /REARM' (again, note the space in the middle of that).
Good luck!