and if you're not careful, you could end up buying a pirated copy of windows.
it happened to me once, and i never bought comps from street shops again.
because you're running a pirated copy of windows, the microsoft support site won't be able to verify whether you're OS is genuine or not. THerefore, all updates will fail. (it happened to me).
and also, things like SP2 won't install properly. (happened to me)
and in the end, i had to re-install windows using a genuine version of windows.
(other symptoms include : unable to access cmd or task manager).