I came across this problem earlier today after upgrading to Vista. I tried to change an artist in my iTunes Library and found that I could not do it. I searched the web and found a great article to help those of you having the same trouble I was having.

Article

If the tips given above do not work, you can always do this...

> CONTROL PANEL
> > USER ACCOUNT
> > > TURN USER ACCOUNT CONTROL ON OFF
> > > > DISABLE UAC
> > > > > RESTART OS

Hope this helps!