Press WinLogoKey+R, type Regedit in the Run dialog and press Enter/Return.
Click Edit > Find, type in itunes6464.msi (or the name of the problem .msi if different), change the Look at boxes so that only Data is ticked, then click Find Next.
You should find it somewhere like:
HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Installer\Products\{Long string of letters & numbers}\SourceList
Right-click on the immediate parent key with the long string of letters & numbers that contains the SourceList with the reference to the .msi file and click Export, save it to say the Desktop with the name iTunes.reg so the data can be restored if removing it doesn't help. Now right click on the key again and select Delete from the context menu. This should stop the Windows Installer from looking for the .msi when you reinstall iTunes.
If the install still won't go through then double click the iTunes.reg file you made to restore the data to the registry, then use the method shown at Install missing components to unpack the installer of your previous iTunes installation and extract the .msi file you require. Double-click this to uninstall the component. There are links to recent builds of iTunes in the Older versions section of the user tip.
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