If a shop sells an brand new computer with a preinstalled operating system, they are required to provide at least a licence key, which should be a label attached to the case, and a cd so that you have the means to reinstall if necessary, (though you may not always get the cd,even though you should)
I would argue that they should provide a licence for you, or you are prepared to contact microsoft, as microsoft take a dim view of companies installing unlicenced software on PCs and then selling them.