If you are making a dedicated Asterisk PBX then follow the excellent guide put together by Tom King called PBX in a Flash Installation(pdf).
If you want to make a dual boot machine between PBX in a Flash and WinXPs for test purposes, modify Tom's directions according to these Dual Boot instructions
If you want to install PBX in a Flash on a USB Flash Drive and boot from it, modify Tom's directions according to these How to Boot PiaF from a USB Flash Drive