The most recent (and likely final) release of iTunes that supports Windows XP 32-bit and Windows Vista 32/64 was issued September 2015. It includes support for iOS 9, though not Apple Music, nor the new iPhone SE. Apple Software Update won't necessarily fetch it for you and may claim you are up to date when you are not. The iTunes download page may not offer it to you either.
Right-click on the downloaded setup file and use run as administrator.
See Troubleshooting issues with iTunes for Windows updates if you have any problems with the installation.
Microsoft no longer support Windows 7 but you might still be able to buy an upgrade. If your hardware has the grunt for it you might be better off switching all the way to 10. If it doesn't now might be the time to invest in a new computer.
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