This solution is the only one worked for me… restoring windows to an earlier point did not work for me. So try the following
1) Click Start button.
into the ‘Search programs and files’ field
3) Right-Click on CMD.exe and select Run as Administrator
net stop sppsvc
(It may ask you if you are sure, select yes)
Note: the Software Protection service may not be running, this is ok.
rename tokens.dat tokens.bar
net start sppsvc
10) After a couple of seconds Windows Activation dialog will appear. You may be asked to re-activate and/or re-enter your product key or Activation may occur automatically.