So as one might expect the Steam servers have been hammered today. And it seems a number of people who have actually brought the retail version, are having Empire Total War get downloaded via Steam instead of actually off the discs.
It took six attempts to get the install to work off the discs when I tried about 2 hours ago. You'd run the installer from the disc, Steam would throw an error saying its too busy to handle the request, and the installer would die. Upon checking the 'My Games' tab you'd see Empire Total War trickling down from Steam, not from the disc.
To sort this out, exit Steam, and re-run the installer from the disc and select the re-install option - if Steam throws another error about being too busy try it again. You'll need a patch downloaded from Steam but that isn't too big.
Trying it on my second machine 30 minutes ago was a bit better, Steam again said it was too busy but upon the second attempt the install worked properly, so it looks like the servers are getting a bit better.
Why bother? Installing from the discs is faster than downloading 15GB off Steam, which at the current rate would takes days...