If you need to install Vmware ESXi (or possibly ESX) 4 on a server without an optical drive – that is, install FROM a USB drive (not “to”), here is how to do in in Windows:

1. Download and extract syslinux: http://syslinux.zytor.com/wiki/index.php/Download

You will want syslinux-3.XX.zip – the latest at time of writing is syslinux-3.83.zip and it works fine.

Extract it somewhere – lets say c:\syslinux

2. Insert the USB drive if you have not already, make sure it is formatted fat32, and make note of the drive letter. For our example, we will use k:.

3. Open a command prompt. (In windows Vista or 7, make sure to Run as Administrator)
Change directory to the place you extracted syslinux \ win32.

cd c:\syslinux\win32
syslinux -m -f -a {drive}:
where {drive} is the drive letter of the USB drive.

4. Extract or copy the contents of the ESX iso image to the USB drive (you can use 7zip or winrar – no need to burn it)

5. rename ISOLINUX.CFG to SYSLINUX.CFG

That should do it.

This is based on info from the following sites – I didn’t really change anything except the version, so if this doesn’t work, they these:
http://www.squishnet.com/?p=17
http://www.vm-help.com/esx40i/ESXi_USB_install.php