Did you already save the old music on your new PC (in any folder)? If not, do so. After that, follow these steps:
1. Change to the Media Center View: Music (or "All media files").
Do you see the missed media files? If not:
2. Click "Add/Edit folders" and choose the folder in which your old music is saved.(Mind attached file scs31.png: Where do I have to click?)
You should see your files now,
BUT: Not in the playlist! (Mind attached file scs32.png: Your files should appear here!)
The Playlist of Last 1000 does only show the latest recordings from Audials! Since your old music has not been recorded with Audials on your new PC, Audials won't show them in the Playlist of Last 1000!
Don't worry! If you want to create a new Playlist change to the Media Center view: "Music" or "All media files" and add songs to a new playlist e.g. via drag and drop. Don't forget to save your new playlist ;-)
If you never worked with the media center before this topic might help you:
The Mediacenter and playing media files .