2. Right-click on the first result (cmd.exe) and click on Run as administrator (confirm by clicking Yes if required).
3. Type
slmgr -rilc
net stop sppsvc
cd %windir%\ServiceProfiles\NetworkService\AppData\Roaming\Microsoft\SoftwareProtectionPlatform
rename tokens.dat tokens.bak
net start sppsvc
4. Restart your computer 2 times.
* An alternative might be to run "sfc /scannow" (without quotes)