Here is something else you can look into...
when you put songs in itunes, it makes a copy of the songs in an "itunes" folder located in your "my music" folder
If you have XP, go to Start > My Music > iTunes > iTunes Music.
If your songs have not been deleted off your computer, they would be in this "iTunes Music" folder.
The files should have the Title of the Artist. Inside that, The title of the album, then the song.m4p/.m4a inside that.
If they are still there, open up iTunes and go to File > Import. Open up the Artist and album you want, select the song, and hit import. The song should now be back into your itunes library.