yes, go into your BIOS/UEFI and make your USB drive as first boot device.
some mother board may have a separate boot menu (usually launch by pressing F8, F9 or F12, depends on brand ) which allow you to pick a boot device, consider using this if you don't want to modify BIOS/UEFI.
MSI should be "Delete" key for BIOS and "F11" for boot menu.
You better check your user manual.
After you have installed window 8.1, your machine can still boot from harddisk if you unplug any removable boot device. Of course you can also go BIOS and change the boot sequence back, it's really up to you.