My MacBook Air developed a fault some months ago whereby clicking the button on the touchpad didnt do anything. Its not a hardware fault as an external mouse had the same problem. So I decided it was time to reinstall the OS.
To boot off a USB CDROM you have to have it all connected and
Power the unit on while holding the Option key (Alt)
It was while I was doing this that the Mac Support Technician gave me another bit of advice which I have copied and pasted below:-
Resetting the SMC on portables with a battery you should not remove on your own
Note Portable computers that have a battery you should not remove on your own include MacBook Pro (Early 2009) and later, all models of MacBook Air, and MacBook (Late 2009).
1. Shut down the computer.
2. Plug in the MagSafe power adapter to a power source, connecting it to the Mac if its not already connected.
3. On the built-in keyboard, press the (left side) Shift-Control-Option keys and the power button at the same time.
4. Release all the keys and the power button at the same time.
5. Press the power button to turn on the computer.
In hindsight, I should have tried this BEFORE doing the reinstall, but its no real big deal for as I had nothing of any importance on there.