It depends on what type of XP license you have. The OEM versions are tied to the specific computer it was originally installed on (for examples the ones that come preinstalled on Dell). Or if you bought an OEM copy yourself, then it is bound to the first computer you install it on (according to the EULA atleast).
If however, you bought a retail-box version, then as far as I know, you have full rights to do whatever you want with that license.
As for if you should include Windows in the price, that depends on how willing your buyer is to pay =P