How to play sega games on pc from a usb?

ok i want to turn a usb into a sega console and put sega genesis roms on there so in turn i can boot the emulator from the usb and access the roms from the usb on any computer all while being able to keep saves from games i know it should not be to hard but just need some direction yeah my puncuation sux its yahoo answers not like im writing a buisness expense layout