You've got a working steam activation key, no need to buy it twice I think.
You don't feed a dog twice a day, don't you ?

Now, I'll first advise you to run Ccleaner to get rid of any temporary files
If you do not have the program, google Ccleaner Sourceforge, it should be in the first three results.
It's free and effective, and it also can get you rid of your passwords, so you better have them written down somewhere safe.

If you have Avast, disable it.

Retry to install.

If all else fail, look into here; steam forum support for shogun 2
http://forums.steampowered.com/forum...lay.php?f=1082


Be mindful that there's a whole mess going on with people not able to play, or able to install the game going on as we speak.
My bet that you're among them.
Since I haven't launch the game for two, three days, who knows, maybe I'll join the gang.