Try repair your Apple Software Update.
Go to START/ALL PROGRAMS/Apple Software Update. If it offers you a newer version of Apple Software Update, do it but Deselect any other software offered at the same time. Once done, try another iTunes install
If you don't see ASU from ALL PROGRAMS, check from your CONTROL PANEL
START/CONTROL PANEL/PROGRAMS N FEATURES/highlight APPLE SOFTWARE UPDATE and click REPAIR button.
Temporary disable your firewall and security software.
Check your windows 7 whether it is a 32bit or 64bit and download the right iTunes version. (START/ right mouse click COMPUTER and select PROPERTIES)
Download iTunes from here:http://www.apple.com/itunes/download/
Donot install, choose save file first. Then install.
Let's check your Windows system.
1. Have you done all the important windows updates and service Packs on your system. If not do it now. START button, type in Windows Update and ENTER. Select 'CHECK FOR UPDATES'
2. Let's check your Windows system files for possible file corruption
SCAN and repair possible Windows system file corruption:
Go to command prompt (START/ALL PROGRAMS/ACCESSORIES right mouse click "command prompt" and choose "Run as Administrator"
let Windows fix any system files that need to be repaired.
Restart the computer if it did actually repair any files.
3. Check your system for possible Malware. But you have to do it in WIndows Safe Mode.
Start your computer in "Safe mode with networking", go to this link download a free version of Malwarebyte.
Install and perform update immediately, then do a full SCAN. Remove malware if it indeed finds any. Restart computer to regular windows to let Malwarebyte complete the removal.
To start your computer in safe mode
Press and hold the F8 key as your computer starts. You need to press F8 before the Windows logo appears. If the Windows logo appears, you'll need to try again by waiting until the Windows logon prompt appears, and then shutting down and restarting your computer.
On the Advanced Boot Options screen, use the arrow keys to highlight the "safe mode with networking" option, and then press Enter. Log on to your computer with a user account that has administrator rights.
When your computer is in safe mode, you'll see the words Safe Mode in the corners of your screen. To exit safe mode, restart your computer and let Windows start normally.