So, you got this PC or Laptop and you're told that it has a brand new and clean Windows installation after formatting the HDD a few days ago... Did you know that there is a command in Windows 7 and 8 from which you can have the precise date of the Windows installation? Check them out!
For Windows 8 open Command Prompt and run
|systeminfo | find /i "Original"|
For Windows 7 open Command Prompt and run
|systeminfo | find /i "install date"|