Posting here this useful (tried and tested) information from How-to Geek:
Finding the Windows Key Without Any Software (Advanced Users Only)
Assuming you can boot your computer without any problems, you can easily create a simple VBscript that will read the value out of the registry and then translate it into the format that you need for reinstalling. We’re not sure where this script came from, but reader raphoenix posted it on our forum a long time ago, so we’re sharing it here for you.
Copy and paste the following into a Notepad window:Set WshShell = CreateObject("WScript.Shell") MsgBox ConvertToKey(WshShell.RegRead("HKLMSOFTWAREMicrosoftWindows NTCurrentVersionDigitalProductId")) Function ConvertToKey(Key) Const KeyOffset = 52 i = 28 Chars = "BCDFGHJKMPQRTVWXY2346789" Do Cur = 0 x = 14 Do Cur = Cur * 256 Cur = Key(x + KeyOffset) + Cur Key(x + KeyOffset) = (Cur 24) And 255 Cur = Cur Mod 24 x = x -1 Loop While x >= 0 i = i -1 KeyOutput = Mid(Chars, Cur + 1, 1) & KeyOutput If (((29 - i) Mod 6) = 0) And (i <> -1) Then i = i -1 KeyOutput = "-" & KeyOutput End If Loop While i >= 0 ConvertToKey = KeyOutput End Function
You’ll need to use File -> Save As, change the “Save as type” to “All Files” and then name it productkey.vbs or something similar ending with the vbs extension. We’d recommend saving to the desktop for easy access.
Once you’ve saved it, you can just double-click and the popup window will show you your product key.
Protip: If you use CTRL + C when the popup window is active, it will copy the contents of the window to the clipboard, and then you can paste it into Notepad or somewhere else.