Its quite easy to boot esx from a usb device or flash card..
First download the .iso, then browse with winrar or similar. See if you can find the “big”.dd file this is the file we need.
i.e. the ESXi 4 file is called VMware-VMvisor-big-208167-x86_64.dd
Next grab WinImage and “restore image to physical harddrive” choose the .dd file the select your usb / flash drive.