"ALSO is it possible to buy a hard drive which is compatible with any motherboard, so that if a computer system malfunctions the hard drive could be transferred to a system with a different motherboard, and function correctly."
You don't say what your motherboard is but unless it something very unique then generally yes you can instal a second hard drive into the PC and use this as a backup device. There are lots of software that will do this automatically so you daughter-in-law doesn't have to remember to take specific actions. The two main types of hard drive are PATA and SATA. The difference being parallel or serial interfaces, so you have to get the right type for your system. There may be disk size limitations if you use Win98 but with XP you are fine.
Also consider an external hard drive, which you connect via USB port. Can be used as above and prices have fallen a lot so that a reasonable size can be bought for around £70.