Every OEM computers come with a recovery disc that contains the
OS to be used for that computer only and the mobo graphics drivers.
It is to be used mainly when your computer crashes and you are
unable to repair and access your operating system then you need to
do a clean format.
For PC builders they will purchase a copy of the OS from retails
and install it on their own.
OS will come with license key for you and you need it to activate
the product and receive the updates from the support sites.
The person who reformatted your computer charge $10 for every
repair that anyone can do with one compy of the operating system.
It can be installed on many computers but once the end user try to
access the internet and update the OS installed he will face
problems, many problems 
Now a copy of original OS (WinXP) costs about $355/-. Divide it by
$10 per service you can do it for about 35 times and by default
without activation the computer can last 1 months. 35x1 is 35
months or 3 years.
mmm..., maybe its worth it after all 
