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.
2) Type: CMD.exe into the ‘Search programs and files’ field
3) Right-Click on CMD.exe and select Run as Administrator
4) Type: 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.
5) Type: cd %windir%\ServiceProfiles\NetworkService\AppData\Roaming\Microsoft\SoftwareProtectionPlatform
6) Type: rename tokens.dat tokens.bar
7) Type: cd %windir%\system32
8) Type: net start sppsvc
9) Type: slui.exe
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.