It doesn't install to the C drive, it just has to put certain files into System 32 folder like every other game, when it does that it gives you the choice of choosing where to install the game.
By the way if you finished the demo and think ok what now.
Open up My Computer on your PC.
Then go here.
D:\Crytek\Crysis SP Demo\Sandbox2 Installer
And install the Sandbox for Crysis. I'm installing it now, for you who don't know what this is, it's usually a modification for games where you can fiddle around with what ever you want and spawn items :D But Crysis is version is much more high tech :P
The CryENGINE® 2 Sandbox 2 can be started by running the
Editor.exe application found in the Bin32 directory for usage on
a 32bit PC or in the Bin64 Bin64 directory for usage on a 64bit PC.
Warning: Running the Sandbox Editor under a 32bit OS can lead to
instabilities with production size levels due to the low amount of
virtual memory available. Though the Sandbox Editor can be used in
this configuration to learn the feature set its use on a 32bit OS
is not supported.