For starters this is still a new science and many people have had good luck with at least one of these methods and others have not.
A FEW THINGS YOU NEED TO CONSIDER IN ADVANCE.
1. The PC has to support booting from a USB flash drive. There may be anywhere from 1-3 items to change in the BIOS to make this possible assuming your BIOS supports it. Some bios's may refer to your flash drive as a USB floppy or USB zip.
2. The USB flash drive must support booting from it in general.
3. The flash drive must contain the boot/system files.
4. The flash drive must have bootsector area. This is done with special utilities.
5. References to "A:" drive lines in the autoexec.bat and/or config.sys files you copy to the drive after you make it bootable may result in errors.
6. You "may" have to format your floppy disk first in WinXP before you create a bootdisk as XP may "not" like working later on with a disk formatted otherwise.