Well there is no real way to transfer music from windows media player to iTunes. The best way to add music to iTunes is by adding it manuaaly.
Here are few steps that might help you a little.
First of all find out where your music files are located. (Most likely they will be in my music folder in your my documents)
Now all you need to do is open itunes.
Open File menu.
Select Add Folder to Library
Locate your music folder and click Ok.
That should add all music back to iTunes.