Quite a few years too late for the Vista to Win7 upgrade, same is now for Win 7/8.1 to Win10 as said above.
If you wanted Win10 would have to buy it.
Win10 is a different look to Vista/7 though so might take your Grandma getting used to.
Vista actually turned out to be not so bad towards the end of it's life with service pack 2. A reinstall of Vista might be an option but not sure if Service pack 2 is still available through it's update system with Vista's extended support ending in April of 2017.
Drivers for Vista might be another problem if you don't have the driver cd. Check online for driver availability at the motherboard's website ie network lan, chipset, sata storage and sound etc. If you still have the motherboard cd that will be handy.
Win10 would be the easiest way since it's more advanced in drivers than Vista/7 are so that part of the hassle would vanish. But would mean spending money for Win10.