Archive iPhone backup in Windows
iTunes for Windows normally maintains one rolling backup set for each device unless you restore a backup in which case it is archived with the date of the restore and a new rolling backup created. On a Mac you can get iTunes to duplicate a backup set, but that feature isn't replicated in iTunes for Windows. With iTunes closed you can select, then copy and paste an existing backup folder set in C:\Users\<User>\AppData\Roaming\Apple Computer\MobileSync\Backup and then give it a folder name that ends in -yyyymmdd-hhmmss. When you restart iTunes the extra backup set will be listed under Edit > Preferences > Devices although it will have the same name as the original it was cloned from. Once you refresh the active backup it should be possible to tell which is which from the time stamps. The behaviour of the copy isn't quite the same as an archive made by restoring, but it should be just as effective if you need to restore from it.
Creating an archived backup is essential before installing beta versions of iOS and should probably be done before major upgrades. Having more than one backup reduces the risk of data loss if you should find that a backup won't restore. You may have to go back to an earlier snapshot of your data, but at least you won't lose everything.
See also Relocate iOS device backups for advice on moving the iOS backup sets to a drive with more free space, and Backup your iTunes for Windows library with SyncToy for general backup advice.