It appears Norton may be causing the problem here.
Temporarily disable antivirus software
Using antivirus software is a smart idea, but some antivirus software can interfere with installing iTunes. You might try temporarily disabling your antivirus software while you install iTunes.
Disable other conflicting software
In Windows, some background processes can cause issues for other programs and processes. These conflicts can prevent applications such as iTunes and QuickTime Player from installing. See "Using MSCONFIG to troubleshoot conflicts in Windows" for assistance disabling conflicting processes.