Easy, copy the game on another hard drive, run Steam offline and reconnect it to your copied virgin version, et voila
Used on Windows 7 64 bit with Firefox 41.0.1
[Edited by B4Marc, 10/1/2015 6:49:57 PM]
Problem is Steam will then lock the game when you go online. Due to their being updates. Then stops you from playing it offline because it knows there is an update. I have tried to do this myself Marc and it has never worked.