If you're curious how old your computer is (or when you last did a clean install), this command line utility that can tell you when Windows was installed.
The systeminfo command gives you all sorts of details about your computer. If all you want is the date Windows 8 was installed, open a Command Prompt and run:
systeminfo | find /i "Original"
For Windows 7, run:
systeminfo | find /i "install date"
The added parameters give you just the original install date. If you've reinstalled Windows it will give you that date instead, but it's still handy.
Find your Computer Uptime and Installation date [Make Tech Easier]