ls -alh /boot/grub #the majority of files in this folder don’t change from the time of install except for menu.lst

ls -alh /restore #BSD

Windows:

Systeminfo #from the command line

- or –

CMD /K WMIC OS GET InstallDate

You’ll see the Operating System installation date (in WMI format). The first eight characters in that output gives you the installation date.

*Windows bit stolen from http://windowsxp.mvps.org/getosdate.htm