"Your system has not been modified" iTunes download error.

Hello, this is my first time asking for support so I hope I provide all the information needed.

Background:

A while back my elder sister went into my computer and messed some settings up because iTunes was being wonky and not updating her new iPhone properly (turns out later the iPhone itself was having issues - not my computer nor iTunes). I used an older version of iTunes because I had an older iTouch and didn't need the newest version, but I didn't have any complaints when she updated it. I come back one day and iTunes isn't working alltogether, and when I plug in my iTouch it tells me there's an error and I need to re-install iTunes (or somewhere along that line, I was stupid and didn't screencap the message). (I asked her what she had done and said "Updated your computer! It hadn't been updated in so long, and then iTunes stopped working! Beats me!" and she can't tell me exactly WHAT she changed and downloaded.) So, I went through the uninstall process and went to the Apple website to download it again, but it kept giving me a "Your system has not been modified" error. This was a long time ago but I came back to my computer to try and solve it again and I've tried to redownload it off the website multiple times but it gives me the same error.

I went into my control panel and "add/remove progreams" to see if anything was left over from the uninstall process but I can't find any iTunes files. Any idea on how to get iTunes back onto my computer?

TL;DR:

"iTunes Installer Completed

The installer encountered errors before iTunes could be configured.

Errors occured during installation. Your system has not been modified.

Please run the installer again, or click Finish to exit."