Booting usb on OLD xp pc

Hi this is my first thread because this has been annoying me so much...

I need to install xp on a computer that wont boot "safe mode aswell" it just restarts and i thought i could just re-install xp on it from usb...apparently this pc is so old that i cant boot from a usb to install xp is there a way that i can make the usb act as a cd-rom so i can boot that way?

p.s i dont have the disk so i need to do it this way somehow....

p.s.s sorry about messyness of post. xD