iTunes library locked or on a locked disk

Hi

I am on a MacBook running Mac OS 10.6.4 and iTunes 9.2 (61). Since a while I am getting an error message while opening iTunes saying, that my library is either locked or on a locked disk. I found some solutions on the net saying you should remove your library file, open iTunes and then move back your old library file to the previous location. That works but my question is why is this error happening in the first place and how can I prevent it in the future. I am sharing this library with a Windows computer on my private network and when I try to open it there this error message pops up as well. I then have to either do the above mentioned or go into my finder on my Mac and change the permissions for the library file to read&write (although in the sharing options the iTunes folder is set as read & write). Why is all this happening. I never had problems with this before and suddenly after a couple of iTunes updates this happens. Also, when I click on the library file on my Mac under permissions it doesn't show my username, only after removing and copying back the library.

Cheers
Martin