USB multiboot-help

I use my flash drive to install new OS.. How to make multi-boot USB? Help me if u knw.. I make USB as bootable using 'cmd' commands.. Do mention any softwares which ll be useful to make multi-boot USB..

multi boot to what? xp and xp or xp and win7?

For nly xp. . . .

For only xp or..

2007xp