JohnJohn_95,
There isn't enough information provided by you to help. It depends on your operating system and iTunes versions. Basically, in past versions of iTunes (early and mid 10's), there were issues with apple software update getting in the way of re-installing or upgrading iTunes. One issue in particular is with permissions to all files and locations involved in the process of re-installing or upgrading. So, it depends on what you have and what you are trying to do. I probably can't help too much at this point, because I have moved on to Windows 8 and stayed with iTunes 10.7.
You could try the following general tips:
1. Make sure you do everything as administrator.
2. Make sure the administrator has full permissions to the installer and all program folders and user settings folders which are changed with the process (do research if you don't know how).
3. FIRST, backup all iTunes settings (all folders which contain, amongst other types: ITL, plist and XML, etc files) (multiple locations).
4. FIRST, backup all iTunes music (all MP4, MP3, FLAC, etc files).
5. Uninstall ALL apple related items (iTunes, apple software update, quicktime, etc) (make sure you have any paid-for-licenses set aside in case they are needed).
6. Delete all apple related program and settings folders (you are looking for a completely fresh install here).
7. Restart if you haven't done so already.
8. Try installing iTunes.
** Copy back the items you backed up, to their rightful locations and replace what is in those locations.
** If you still have issues with iTunes, then you may have to do some research
** You may have to do some research to see where all of the folders and files are on your individual system. It can be different for each person.
** I of course, can't guarantee success or that no more issues happen.
I really can't help any more than this, because each person's system and ability is so different.
Good luck, thebestjustme