iTunes 12.3 on Windows 7 stopped working on startup.

I was having this problem as well and spent numerous hours on the phone with Apple trying to rectify it. Anyways, it is caused (or at least in my case was caused) by the fact that I was running Windows 7, but did not have Windows Service Pack 1 installed with my Windows 7. Once we installed Windows Service Pack 1 and rebooted the PC, the problem was fixed and iTunes ran perfectly fine.

You can find Windows SP1 here: http://www.microsoft.com/en-us/download/details.aspx?id=5842

If you have Windows 7 (64-bit) you need to download this version: windows6.1-KB976932-X64.exe

If you are running Windows 7 (32-bit) you need to download this version: windows6.1-KB976932-X86.exe

Hope that helps out.

Also, maybe the steps suggested in this article may help you:

Fix unexpected quits or launch issues in iTunes for Windows - Apple Support

Unfortunately, the steps at that link do not correct the problem as it is a problem with missing Service Pack 1 for Windows 7.

During my trouble shooting with Apple over the past few days, we went through all of them methodically with no success. The only way to fix the problem is to install Service Pack 1 for Windows 7. If this is already installed on your PC, then you may be having an entirely different issue with your iTunes.

However, the APPCRASH error related to Fault Module Name: CoreAudioToolbox.dll stems entirely from not having Service Pack 1 for Windows 7 installed on your PC. Once you install Service Pack 1 you should no longer have any problems with iTunes.

In order to determine if you have Service Pack 1 you need to click the blue start button in the bottom left corner, then click Computer, than right click on the C drive and click properties. When that screen opens up, on the top it will say Windows Edition and underneath it will say Windows 7 Ultimate and it will also say Service Pack 1. If it does not say Service Pack 1 then it is not installed and you need to follow the steps I listed above to install it.