How to cleanup orphan itunes library entries.
iTunes libraries will flag orphan entries with an exclamation mark!
These entries can be cleaned in two ways:
A) Create a new library:
Start iTunes by holding the shift key (keep the shift key pressed, while you select and start iTunes). iTunes will prompt you to create a new library (or choose an existing one).

B) Use a 3rd party tool:
iTunes Library Updater is a free utility to synchronize the iTunes database with your media collection. I run it with the ‘Clean orphaned entries’ option only:

note that .NET Framework must be installed to run this programme
iTunes 9 – orphan entries