It is a big issue for a huge number of users. There is no clear fix yet, but some users have had success with some of the solutions on this thread. Good Luck, Hopefully Apple will step up soon.
Here is what worked for me for both 10.5 and 10.5.1 after hours of frustration with Itunes stopping and shutting down in the middle of a syncor tranferring Apps downloaded on my Ipad2 or Iphone 4:
Start iTunes in iTunes Safe Mode:
1. hold down the Shift and Control key together and click to start iTunes.
2. when you see the iTunes icon in the task bar, click on it.
3. you will get a box that says "iTunes is running in safe mode. Visual plug-ins you have installed have been termporarily disabled." [I don't know what plug-ins I installed but you should clearly see that these so-called visual plug-ins are likely the cause of your issues];
4. click Continue to open iTunes.
You could already have your device connected to your computer or do it after opening iTunes in its safe mode.
Now press your sync button and everything should work fine.
I also disabled Genius in iTunes preferences but I'm not sure if this is one of the issues causing these types of problems.
I hope this works. This may be the workaround for all of us Windows 7 64-bit folks.
My iTunes was working just fine and I fnally gave in to the upgrade requests. Now, it won't even open. I have tried rebooting, uninstalling and reinstalling (multiple times), upgrading, downgrading, holding the shift+control keys while starting iTunes (still won't open). I have Windows 7, 64-bit. I am so frustrated. I hope all my music is on my iPod - it needs to be recharged and I'm afraid to hook it up to the computer while iTunes is out of commision. I don't want to lose my entire library.
*rips hair out*