Here is a way to know when you installed your system for the last time. The hocus-pocus is only true if you didn't modify your root partition ("/") since the last installation.

Let's go :

dumpe2fs -h /dev/DEVICE | grep 'created'

Replace DEVICE by the name of your root partition (hda1, hdd1, sda2...).

Here is the output (we use grep because it's faster) :

Filesystem created:       Fri Apr 18 13:33:51 2008

Fun no ?