In my personal opinion you should just respect your fathers wishes. He doesn't want anything fancy, he isn't going to be doing anything demanding, he wants familiarity and a PC. Win7 isn't something that is going to explode 2 minutes after installing it, nor is XP. If he can save $400.00 and I'm assuming he has a budget if he feels it's too expensive, then he can use that money he has saved to enjoy his retirement and indulge in other hobbies. *Personally* I think a good son would try to accomodate his needs. my 0.000001 cents. 
Macs are not the alpha and omega, and they're not for everyone.
Macs are not the alpha and omega, and they're not for everyone.