2 main options.
1) You manually select the boot disc. When you power on your PC it will say something like "press f8 for boot options" (might also be f12 or something else entirely). From there you can select the external, a DVD, a the internal hard drive....
2) You install some kind of jump loader. Traditionally people would do this to boot a PC from a device that the BIOS did not support, mainly USB. However you can install a boot loader on a device of your choice and then get it to boot from that, it will then display an options screen and allow you to pick between them.
If you are the only one using the machine and you only want to go into windows occasionally then I would probably just opt for 1). If multiple people want it and they are not as tech capable then have fun getting 2) sorted. Sometimes it is painless but other times it gets very annoying.
If you prefer a Windows based bootloader then I like
http://neosmart.net/EasyBCD/
If you prefer a Linux based bootloader then you have a few options, I am not sure what Ubuntu is doing these days (newer versions of windows and UEFI changed a few things) but normally it is Grub (
https://help.ubuntu.com/community/Grub2 ) that is used. You have probably already seen something like this where it says "ubuntu some version" and on the next line is says "ubuntu some version (single user mode)" and gives you a few seconds to choose.
Both will allow you to choose between Linux and Windows and whatever else you want in there. I like and use both for various things.
