How come my computer boots from my external USB drive instead of my C drive?

Q: Hey i have windows xp Professional Intel Chip and Every time my computer restarts its booting from my external hard drive that i have pluged in that is usb. I would like for my computer to Boot from C drive instead of USB is there a way to make it so it wont boot from usb and boot from c drive only way i can do it is every time i restart my computer i unplug the usb and it will boot from c drive but other then that it wont.

Thanks