Errors occurred during installation, install failed.

Question

So recently I got into a bit of trouble when my web browsers got hacked and started installing some adware on my computer... I'm still getting that sorted out, but I think while I was going through my "Common Files" folder and deleting things that looked suspicious, I accidentally deleted some kind of file for Apple's many programs that install with iTunes.  I realized my mistake when, at the next reboot, Windows gave me an error that some program could not start due to a missing .dll file and then gave me the program name, which as it turns out was Apple Application Support.  I uninstalled the support program to attempt to correct the issue, but when I did it only broke things further!  iTunes claimed that it could not work without the support software, so I tried uninstalling and reinstalling iTunes to get the Application Support program back.  Sadly, this did not work either as iTunes did not uninstall correctly, freezing windows explorer.  Then when I downloaded the install file to try again, it gives me an error almost immediately saying "Errors occurred during installation, your system has not been modified.  Run the installer again."  (I'm paraphrasing)  When I run it again, the same thing happens.
In conclusion, it appears that by deleting one simple file, I have broken iTunes permanently and I can no longer use it on my computer.  Ugh.  Can anyone help me get it back?  I can't believe I'm even asking this... I'm not really an apple fan but I do use it to put videos and stuff on my iPad and I have a bunch of playlists in there that I've spent time organizing, so I still use it as my "main" mp3 player.  This is very inconveinent!

Answer 1

Windows gave me an error that some program could not start due to a missing .dll file and then gave me the program name, which as it turns out was Apple Application Support.  I uninstalled the support program to attempt to correct the issue,
Let's try a standalone Apple Application Support install. It still might not install, but fingers crossed any error messages will give us a better idea of the underlying cause of the issue.
Download and save a copy of the iTunesSetup.exe (or iTunes64setup.exe) installer file to your hard drive:
http://www.apple.com/itunes/download/
Download and install the free trial version of WinRAR:
http://www.rarlab.com/
Right-click the iTunesSetup.exe (or iTunes64Setup.exe), and select "Extract to iTunesSetup" (or "Extract to iTunes64Setup"). WinRAR will expand the contents of the file into a folder called "iTunesSetup" (or "iTunes64Setup").
Go into the folder and doubleclick the AppleApplicationSupport.msi to do a standalone AAS install.
Does it install properly for you?
If instead you get an error message during the install, let us know what it says. (Precise text, please.)