Well at the risk of being a little controversial, you might consider building a PC for your Dad. It's actually much easier than it sounds, and you will get a better value PC :) And you'll more of an idea of what to do later on if your Dad does anything to damage it!
This site is a good place to start click here
I've used this site's forums to put forward the specs for a computer for my Dad (under SemperSi), and there are a lot of people who will check the proposed system for you and advise.
If you really don't want to build then the Dell range are generally pretty reliable and good...