Too bad they are so set on using windows. although for someone that knows how to take care of it, windows can be fine, for those who don't, it is difficult.you need to set them up with a good firewall and anti-virus that will keep their computer running clean but one that even a novice can use. (ZoneAlarm, for example, while considered one of the greatest firewalls, has way to many prompt.) also, make an image so that you can quickly restore thier computer to a good state should something happen. also, partition thier drive to atleast 2 parts;make c like 20GB and the rest as the My Documents root so that all thier data will go to D and so you can then easily repair thier computer. anyway, best of luck.
P.S.: BTW, why are they so sure that they don't want a Mac? the macs are significantly better for older people as the UI can be adjusted more to suit thier needs. and they do not need a two button mouse. my grandparents survived perfectly well on thier PC never even knowing the purpose of the right button on the mouse.