Before you do this make sure that her PC more than meets the minimum specification for Xp and check that her peripherals have XP drivers available for them, if not then you may need to buy a new printer or scanner and that can add to the expense. The cheapest way of getting XP is to buy the OEM version such as this click here however 2 things I should point out, in order to comply with the licence terms you would need to buy some hardware, this can be as simple as a mouse and the other thing is that because its OEM once its installed on your mothers PC you can't buy a new PC and put it on that. Its tied to the machine its put on. If the PC will last for years then ok but if she may buy a new PC in a year or so it may not be such a bargain. Also, perhaps not so important Microsoft do not provide support for OEM versions you are expected to sort it out yourself. But then thats what this Forum is a real help for.