"Computers are often sold without back-up discs to keep the price down, as individual Ms Registrations wouldn't be needed. "
Not quite, the only reason it keeps the price down is through the cost of the physical media (the CD or DVD), the cost of the license for windows still has to be paid.
A lot of manufacturers will have a utility on the computer that will enable you to create a set of restore disks using blank CD/DVD's but as you've not said what make of computer it's hard to advise if this would be possible for you.