If you need to downgrade and or uninstall the current iTunes installation on your system, use iTunes Eraser first. Then you can install any iTunes version compatibile with your operating system.
iTunes version 7.0-9.2.1 is compatible with Windows XP SP2-Windows 10 32 bit. iTunes 7.6-9.2.1 and newer also support Windows XP SP2-Windows 10 32 bit and 64 bit.
iTunes version 7.0-9.2.1 is compatibile with Mac OS X 10.4.10-Mac OS X 10.9.5, PPC and Intel.
iTunes version 7.3 was the first version with iPhone support. iTunes v7.3-7.3.2 can restore/sync/update an iPhone on firmware 1.0-1.0.2 offline.
iTunes v7.4 was the first version with iPod Touch support. iTunes v7.4-7.4.3 can restore/sync/update an iPhone/iPod Touch on firmware 1.0-1.1.2 offline.
iTunes v7.5 can restore/update/sync an iPhone/iPod Touch on firmware 1.0-2.0 beta 3 offline.
iTunes v7.6 introduced verification with Apple servers for restores and doesn't work offline like iTunes version 7.3-version 7.5 do.
iTunes v7.7 was the first with iPhone 3G support. It supports firmware 1.1-iPhone OS 2.2.1.