You may just need to tell your laptop to boot from the USB first (and not the hard drive), there might be a boot menu available, or a setting in your BIOS to change.
Or the USB may not have been created successfully. If you're stuck on old windows XP the Universal USB Installer from pendrivelinux.com should still work, and it was very successful.
Or, your it's possible your old laptop is unable to boot from a USB... I think there were old ways to install linux without using a cd/usb, but they involved a box full of floppy disks... a working usb cd/dvd reader, or a bootable usb would be the easiest. WUBI used to install Ubuntu from "inside" old windows versions, but it wasn't perfect & is no longer supported.