Technically yes, it is, but it's not supported, and if you're thinking you can just take it to any random computer and boot a copy of Windows, you're going to be sorely disappointed.
For the most part, Microsoft has been working hard to make this sort of thing increasingly difficult since Windows 95. Each successive version of Windows makes it that much harder.
You CAN install numerous Linux distributions onto such devices and boot it without issue, so long as the computer's BIOS supports such.
am thinking to install a windows xp on the 8gb USB flash disk. is it possible.? how.?