You can find your new songs and videos by clicking the Purchased playlist in the iTunes Source list. You can work with the Purchased playlist as though it were any other playlist. That is, even if you delete a track from it, the song itself still remains in your iTunes library. And behind the scenes, the corresponding music file stays in your My Music→iTunes→iTunes Music(Home→Music→iTunes→iTunes Music) folder.
If you're going to back up anything in your iTunes library, make it the Purchased list. After all, you paid for everything on it and if your hard drive goes south, that particular financial investment is toast. Lucky for you, iTunes 8 and later has a built-in backup tool that can archive copies of all the files you bought from the iTunes Store.
There's more information about backing up in Chapter 5, but if you're in a hurry, choose File→“Back Up to Disc”. A box pops up to walk you through the process; make sure you have a stack of recordable CDs or DVDs handy.