This might sound like a little bit of a crazy question, but how can I find out (hopefully via an API/registry key) the install time and date of Windows?
Open command prompt, type "systeminfo" and press enter. Your system may take few mins to get the information. In the result page you will find an entry as "System Installation Date". That is the date of windows installation. This process works in XP ,Win7 and also on win8.