Once it happened to me except it was looking for my external storage device.
What I had to do was delete all of the songs in the Itunes interface (the actual itunes window), then go in the My Music > iTunes (or wherever your music is located), and re-drag those songs to your itunes. Should work afterwards.
My problem: when I plug my iPhone into my Windows Vista system, iTunes pops up and begins syncing. Everytime I do this, iTunes attempts to access my NAS drive, which takes about 30 seconds to power up and "ready" the two drives in the NAS.
At one time, I had all my mp3 files on the NAS. But now, I have moved the mp3 files I want sync'd over to my internal C: drive, and have pointed iTunes to drive C: for the mp3 files.
But iTunes still keeps trying to access my NAS drive. How do I make it stop accessing the NAS drive (which happens to be drive Z:).