Check whether iTunes works in safe mode.
To boot into safe mode follow the step mentioned below.
o Restart your computer and start pressing the F8 key on your keyboard. Select safe mode from the menu.
If it works in safe mode then perform a clean boot.
1. Steps to perform a Clean Boot:
o Click Start on your Desktop.
o Type msconfig in the Start Search box and then press ENTER (If you are prompted for an administrator password or for a confirmation, type the password, or click Continue)
o On the General tab, click Selective Startup.
o Under Selective Startup, click to clear the Load Startup Items check box.
o Click the Services tab, click to select the Hide All Microsoft Services check box, and then click Disable All.
o Click OK.
o When you are prompted, click Restart.
To return your computer back to Normal state:
o Click Start on your Desktop
o Type msconfig in the Start Search box and then press ENTER.
o Click the General tab.
o Click Normal Startup - load all device drivers and services, and then click OK.
o When you are prompted, click Restart to restart the computer.
For further details check the article mentioned below.
How to troubleshoot a problem by performing a clean boot in Windows Vista or in Windows 7
How to perform a clean boot procedure to determine whether background programs are interfering with a game or a program that you currently use
Try downloading iTunes and QuickTime separately.
Make sure that you uninstall all the applications related to apple from "Add and Remove programs" and try deleting all files related to QuickTime in "C:/Windows/system32" and then reinstalling iTunes and QuickTime separately.
Follow the steps mentioned below.
1. Uninstall QuickTime and iTunes.
2. Reinstall QuickTime ONLY. Once complete make sure the program opens. If it does close program.
3. Install iTunes ONLY
To download QuickTime. Check the link mentioned below.
To download only iTunes. Check the link mentioned below.