you don't, each time you install software instead of just spam clicking next next next next next finish and getting all kinds of unwanted toolbars and crap, look for a option that says advanced or custom at the beginning of your installation. Once you click advanced or custom install you will be able to select the install directory, then you can point it to you 1TB HDD. Certain things like windows updates, drivers etc... cannot be placed on any other drive but your SystemRoot.
I apologize, I misread the OP
popatim is correct, each browser is gonna have a different procedure for changing the default download directory but they shouldn't be that hard to figure out.