Copy the text below into notepad, then rename the .txt extension to .vbs
Then double click the file and wait a bit, it will do it automatically for you (you'll see it working, be patient).
One of the lines has XXXXXXXXXXXXXXX, change this to your new product key, no dashes before running the file.
Dim oRegShell
Dim oShell
Set oRegShell = CreateObject("Wscript.Shell")
oRegShell.RegWrite "HKLM\Software\Microsoft\Windows NT\CurrentVersion\WPAEvents\oobetimer", 0, "REG_BINARY" 'Clear WPA key so that msoobe will run
Wscript.Sleep 3000
Set oShell = CreateObject("Wscript.Shell")
oShell.Run "%systemroot%\system32\oobe\msoobe.exe /a" 'Run activation program
Wscript.Sleep 8000
oShell.AppActivate "Activate"
Wscript.Sleep 5000
oShell.SendKeys "%t%n" 'Select second option "activate by phone" and hit next
Wscript.sleep 3000
oShell.SendKeys "%p" 'Change product key
' Maybe some wait time while key is generated
WScript.Sleep 13000 'multiple by 1000 for seconds
oShell.SendKeys "XXXXXXXXXXXXXXXXXXXXX" 'Here you enter the new product key, without dashes.
oShell.SendKeys "%u" 'Hit "update"
wscript.sleep 3000
oShell.SendKeys "%{F4}" 'Close Activation program.