That's a bit of a puzzle then, Steeve. I suppose you could go back and look at the file creation dates of certain files. Here are files I found on my computer (which, admitadly, is not upgraded to Windows 8) that were created the day I installed Windows 7:
You could also look in C:\Windows\inf\ and sort by date created. You'll have a huge block of .INF files created on 7/11/2009 (when those files were packaged for the public Win7 release) and then another big block of files after that with the date you installed Windows (as extra necessary files were downloaded). I have a block of 12 files created within 2 hours of the time I started the initial Windows installation.