The usual way of implementing a dual-boot system is by software. During the boot sequence the user selects the required operating system from a list. This is fine until one or other of the installed operating systems requires upgrading to a later version. Can we be confident that the dual boot will still work after the upgrade? At the very least we would be advised to backup everything.
There is an alternative which avoids all such worries; instead of switching between operating systems in software, use a hardware switch. Imagine a simple toggle switch on the front of your machine: UP for Ubuntu, DOWN for Windows – select the OS then power up.