if you were to look on the box, you would see that it can be installed off the original disk three times. not sure what one would do after that... might want to call m.s. and see if they have an extension or if you have to rebuy it. bought mine and installed it on the xp machine till it died due to sp2 and then reinstalled onto the new vista os. still have one left, i guess... in any event, m.s. will be happy to answer a question or two from a customer...