If you want NO problems with a new computer, Dell is really the only way to go. I've never built a PC from scratch that had no problems, even though it's way more powerful and cheaper than a Dell for the performance/price ratio.
If you really want to build a solid Athlon XP, system, I recommend the following:
<UL>
<LI>Abit NF7-S Rev. 2.0 (Extremely good on-board sound, USB 2.0, on-board LAN)
<LI>AMD Athlon XP 2500+ Barton (1.8 GHz)
<LI>GeForce 4 TI4200 64 MB
<LI>40 or 60 GB Maxtor 7200 RPM HDD
<LI>US Robotics Standard 56k Modem
<LI>CyberDrive 62x24x52 CD Burner
<LI>512 MB Kingston PC2700 DDR-RAM
<LI>Generic Case with 350w PSU
<LI>Generic Floppy Drive
</UL>
That should put you at about US $500 max. But like I said before, this computer would be better than what Dell has to offer, but it has no warranty, customer support, or guarantee of hassle-free use. If you put it working 100% of the time at the top of the list, then I wouldn't go custom-built. Fixing a built from scratch PC is half the fun (sometimes), but in your dad's case it doesn't sound like he wants to be fixing another computer.
Good luck!
-Lasereth