windows genuine disc re-installation.
you can install upto 3 times in 150 days.
if you need to do it more than this in the 150 day period then you have to phone ms for an activation key code.
the corporate version is when you dont have a legal copy,and you take it to your computer shop to have it reinstalled, usualy costs between £25 > £50.
but you wont be able to have the updates,as said before ms declares this as an illegal copy, but there was a dispute a few months back about ms letting these people have the updates,as they have paid there computer shop for it,even though it is a generic key code, they pay ms for a volume licence, or called something simmilar.
