use the additional drive. It is up to you to place things on it.
When you download something you must redirect the download to the second drive. Once you develop the habit it really isn't difficult.
Here's a link to a MS article to relocate your My Documents folder(s) to that drive and make it the default.
Since I got iTunes my memory was being eaten alive with podcasts. So my computer store folks suggested an additional hard drive on my computer. I am starting to get a low memory message, and yet nothing is being stored on the 80 GB HD. I don't know what will make the computer start recognizing and using the new larger storage space. I have transfered some stuff "by hand" but it's a very cumbersome process.
I have an HP Pavilion with Pentium 4 1.80 ghz x 2
and I run Windows XP. The hard drives are the 40GB original and an 80GB addition.