When a computer�s power is switched off the operating system is no longer maintained in RAM (random access memory). When power is restored it is necessary to reload the operating system from permanent memory (the hard drive) to RAM where it can actually be used.
A problem in developng computers was how to start one the first time - you needed to run a start up program but couldn't because the computer wasn't running.
Someone described it as like "trying to pull yourself up by yout boot straps (laces)"
That is where the expression comes from to "boot a computer" it's not actually to do with kicking one - but that's good too! :c)