...you will tell him to get a professional to do this for him. Whether he understands it or not, your father's livelihood depends on having computers that are up and running, and from the limited picture you've given us, it sounds like he only thinks about information technology when something is going wrong.
Simply replacing his current hardware with newer gear is just kicking the problem down the road. In the last 15 years, there have been significant advances that he should consider taking advantage of, because they can make his business run more efficiently. But the only way to determine the "right" solution is to have someone who understands your father's business design a solution for him. A good place to start would be with the vendor who sold him his current setup, if they are still in business. They can most likely recommend a suitable hardware platform and assist with data migration to a newer, supported version of their software, and provide some sort of service arrangement that will ensure that these systems are maintained, not just used.
If you're bound and determined to do this yourself, the recommendations about virtualization are good ones - you can build a couple of VM images that you can backup to a USB flash drive nightly, or even better, several flash drives - learn about backup rotation schemes and design one that gives you the ability to recover two weeks worth of image data at least.
Up thread, someone said that if you do this, you will be supporting it for the next 10-15 years. You need to keep that in mind when deciding how to proceed. Best of luck!