Here is an example of the great kind of information you can find in the VMware Community Forums. Paul provides complete instructions on how to install ESX Server from a USB flash drive. I’ll give you the requirements for this here, but you’ll have to read the full post yourself here.
Requirements:

- You will need at least a 1 GB USB flash disk.
- You must have both the ESX 3.x CD-ROM and ISO image handy.

- You will need an ISO image extraction tool such as WinISO, MagicISO, or even WinRAR.

- You’ll need SYSLINUX, a Linux boot loader for removable media. Get it here:

http://www.kernel.org/pub/linux/utils/boot/syslinux/syslinux-3.36.zip

- These instructions are based on a Windows PC for simplicity. If you’re a Linux guru, then you can obviously do this under Linux. You’d need to know how to mount .ISO images in order to extract files, etc.