1.After logging in (you must be logged in as an administrator to do this, so do so if you’re not already), you’ll be asked to enter a key or use Windows in reduced functionality mode. Click the option for using reduced functionality mode.
2.In the web browser you’re presented with, type “C:\Windows\System32″ in the address bar and hit enter.
3.Find cmd.exe in the System32 folder, right click on it, and choose “Run as Administrator”.
4.When UAC prompts you to elevate permissions, click allow.
5.Type “slmgr -rearm” at the command prompt and hit enter.