Press Ctrl-B to enable the menu bar, you'll then have File > Add Folder to Library... available. You can also copy existing media to your iTunes library by copying it to the Automatically Add to iTunes folder within iTunes' folder hierarchy.
To recover playlists, etc. you need to retrieve the iTunes database (iTunes Libarary.itl) from your old computer - a complete backup of an iTunes library includes this as well as your media files. Ideally, you should have everything under a common folder - C:\Users\username\Music\iTunes if you're using the default location, X:\iTunes (where "X" is any drive letter) if you have the library on another drive. The ideal structure is like this:
so that you can create a backup of the iTunes folder and everything you need - either to move your library to another computer, or to create/maintain a regular backup against PC/disc failures - is in one place.
If you did backup and restore your iTunes Library.itl file via Carbonite, or can access and copy it from your old PC, then make sure its in the correct location with respect to your media files, hold down SHIFT when you start iTunes, and when you get this prompt:
click on Choose Library..., navigate to the folder containing your library, select the iTunes Library.itl file (which may just show as "iTunes Library" if Windows is set to not show file extensions) and click Open.
