You need to either change the boot order in the BIOS or hit the temporary boot menu key as the computer is starting up (usually F10, F11, or F12).
You should be able to reinstall as often as you'd like, but you may need to call in to activate. If it's OEM, it's tied to motherboard. Retail can be moved to other hardware. (That's for Windows at least, not sure about Office but would guess the same.)