JiK - here in Aus, secondhand machines that are "ex-office" machines - some of them ex-Govt Dept, come with a label with a Win XP (or Vista) serial number affixed - the reseller can hand out an XP or Vista Disk for the licenced OS - unless it is one of those arrangements where no physical disk is involved, the OS being only supplied on a 'back-up partition' from where you have to burn your own disk if you want the OS SW on a disk separate from the PC (this is most common on laptops).

XP will die (the end of Xp support is the same), but there has always been some legal contention here that MS does not have the legal power to stop secondhand machines being resold with the legal SW originally provided on the machine (MS force their resellers to put a MS OS on all machines they sell new).