OEM Windows XP
You can install & then activate on one PC only, after which it will not activate further on any other PC
Retail Box
You can install it on your PC & activate. If in future you buy a new computer you can move the license to that new computer after which the current computer will be de-activated.
It will still work, just that you won't get genuine Windows Updates etc. Cos once activated on new PC the old PC data on MS servers will be deactivated.
Go for OEM if you plan to use it in one computer only.