That depends ...
Prerequisite would be that the machine's BIOS supports
booting of USB. If it doesn't you're at loss.
And then of course whether XP does ;) ... which I don't
know, but then, hey, this is a Linux-forum! :}
To get linux booted of it you'd have to build an initrd
with all the required drivers (both your usb-chipset
support and scsi-support built into the kernel).
In other words: if it DOESN'T work it's not a lilo-problem ;)