Well, I had the same problem and none of those solutions solved my problem, but I solved my problem by my own. That's it:
On Windows 7/8/8.1
4. On desktop go to your "YourUserNameFolder" > "Organize" > "Folder and Search Options" > Mark "Show Hidden Files, Folders and Drives" > Unmark "Hide protected operating system files" > OK;
5. 'AppData" > "Roaming" > "Apple Computer" and move this folder to desktop. (This step backs up all your content from iTunes);
1. Install Revo Unistaller (The paid version is very recommended);
2. Uninstall iTunes using Revo, removing completely any trace from the installation of it (Use the advanced mode);
3. Reboot your system;
4. After the system restarts, open "Computer" > "Program Data" and delete the folder "Apple Computer". (Don't delete that one you that moved to desktop);
5. Reinstall iTunes and don't open it, after the install;
6. Reboot your system again;
7. Now go back on "AppData" > "Roaming" > "Apple Computer" and move back the that folder that you moved to desktop.
8. Now your iTunes should work again.:D