You can try downloading 7-Zip (free), or a free trial of WinRAR, and unpack the iTunesSetup.exe or iTunesSetup64.exe file into its components then try installing them individually in alphabetical order (don't try to install SetupAdmin.exe which drives the original installer, or iCloud.msi unless you have Vista). You may get a more useful error message regarding any problems during the install.
If you move the components to the root of the C: drive, these commands can be used to log the installs of individual components.
msiexec /i C:\AppleApplicationSupport.msi /l*v C:\Log_AAS.txt
msiexec /i C:\AppleMobileDeviceSupport.msi /l*v C:\Log_AMDS.txt
msiexec /i C:\AppleMobileDeviceSupport64.msi /l*v C:\Log_AMDS64.txt
msiexec /i C:\AppleSoftwareUpdate.msi /l*v C:\Log_ASU.txt
msiexec /i C:\Bonjour.msi /l*v C:\Log_Bonjour.txt
msiexec /i C:\Bonjour64.msi /l*v C:\Log_Bonjour64.txt
msiexec /i C:\iTunes.msi /l*v C:\Log_iTunes.txt
msiexec /i C:\iTunes64.msi /l*v C:\Log_iTunes64.txt
You can triple-click a line to select all of it, then press Ctrl+C, WinLogoKey+R, Ctrl+V, <Enter> to execute it.
Which components you have depends on whether you've unpacked the 32-bit or the 64-bit installer.
The log files might provide further clues as to what is going wrong.
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