Now, when I click on iTunes, a notice pops up that says "The procedure entry point AQOfflineMixerNew could not be located in the dynamic link library CoreAudioToolbox.dll."
Taken at face value, you're having trouble with an Apple Application Support program file there. (Apple Application Support is where single copies of program files used by multiple different Apple programs are kept.)
Let's try something relatively simple first. Restart the PC. Now head into your Uninstall a program control panel, select "Apple Application Support" and then click "Repair".
If no joy after that, try the more rigorous uninstall/reinstall procedure from the following post: