I'm cross-posting this, because there's a lot of people having this problem w/ the same questions in different places. What's your current OS system?
I just had this problem while using my MacBook w/ Snow Leopard 10.6.3. Then I checked "Software Update" (on the Apple Menu), it found a free upgrade to 10.6.8, and the problem's gone. Check Software Update to see what kind of system upgrade you can install for free, and that may well fix it. (Upgrading to Snow Leopard only costs $20.) I think it's a new iTunes issue with older systems, but if you do whatever free upgrade Apple has for you, it might well work.