^
There are a couple requirements as having the right size usb, right information, right computer, etc.
From the top of my head, I can tell you that you need a pretty recent pc that isn't too old. Most of the recent pc's now can boot from usb and the older ones usually can't, or require a bios update possibly. So first and foremost, you need to find this out.
Another, the flash drive, whether it's big enough because xp takes about 1.5gb i believe.
Another, depending on the way you do it, you might need an XP disc with SP2 preloaded already and it'll probably be best to have SP2 updated original OS already installed so there wouldn't be any conflicts or anything.