Did you tried to uninstall it completely. Just try to uninstall it with any third party application like revo uninstaller pro or iobituninstaller and remove it completely from your computer.
Once you removed it completely, Restart your computer then follow the below steps :
1. In run, type regedit
2. Once your got Registry window, press F3 and type apple and hit enter ( make sure that you took the backup of registry before doing this )
3. Delete the file which you got in search.
4. Follow this procedure until it says no more files or similar message like this
5. After that restart the computer once again
6. Then download the latest setup file of iTunes from apple website
7. Install it with option Run as administrator by right click on it (except Windows 'coz in windows XP, it will automatically run as administrator)
8. Hope it will Work. Good Luck.!!!
Note : Make sure that you following all these steps in administrator account
To activate administrator account, you need to run the cmd as administrator by right click on it
type net user administrator /active:yes and hit enter
Log off the current account and login with administrator account..
Good Luck Friends.!!!!