If you're referring to XP, your concern would be with product activation.
You can install XP on any number of machines, but you're only supposed to
activate it on one at a time. (The rule: one paid license per machine.)
http://aumha.org/win5/a/wpa.php
What you can do with XP is, at least in principle, dependent on what type
of license you get.
A retail license (full or upgrade) can be transferred to a new machine any
number of times. An OEM license is supposed to be only installed on one
machine, ever, but I don't know how that machine is defined. (If the hard
disk dies, I believe that Microsoft will give you an activation code for a
re-install.) I suspect that, in reality, you could install an OEM license
on a 100% new machine, although you might have to be less than truthful in
describing it as a "repair". (I don't know from personal experience. My
only XP license is a retail upgrade one, which has been installed in a
series of systems, all within the one license/one machine limit.)
There are also manufacturer's restore disks, and BIOS-locked OEM copies,
with different limits. I have no experience with them. If you're not
getting an OS with an appliance PC, you shouldn't get either of those.
Product activation is a bit of a pain, but if you have to re-activate by
the slowest means (telephone), it still takes only about five minutes (at
least in the US, by toll-free call).
I suggest that if you're given a choice between an OEM version and a
retail one, get the retail. It will cost more, but it may save you some
future aggravation.
HTH.
Bob Knowlden
Address may be scrambled. Replace nkbob with bobkn.