Privileges Error

I persistently get a dialogue box that pops up, several times per session, saying:

"The iTunes Library file cannot be saved. You do not have enough access privileges for this operation."

Hit OK - the dialogue box goes away, only to return a few minutes later.

My AppleTV has unable to access iTunes on my PC via Home Sharing since this happened - in all other ways networking and functionality seems to be normal.

Can anyone cast any light on this niggle?

AndyK