The little circle slash is the Windows standard icon for the window / application you're dragging over will not accept the file.
So, the sequence of events is that you had music in iTunes. You put it on your iPod. You backed it up with another program from the iPod to a DVD using 3rd party software. And now, even though these same songs where in iTunes previously, you can't add them back into iTunes.
1. What are the file formats, AAC, MP3? What's the 3 letter extensions on the files on DVD?
2. What program was used to originally encode these files? Were they downloaded from the internet? Were they ripped in iTunes? Were they ripped in Windows Media Player?
3. Probably the thing to do is send one of these files to
the apple moderator.
iTunes will not open malformed MP3 files and that probably what you're running into. You've run these things through at least 1 shareware program it looks like and they could have easily been corrupted. The most common corruption is malformed ID3 tags, particularly multiple tag headers or tags that are too big. Sometimes you can, with ID3 tag editors, go in and remove the tag and iTunes will be happy with the file, but you would have to re-edit the tag information manually.