Hi, Susan,
Interesting thread, and lots of good, informative answers.
One more thing I'd like to add is that if you (or your dad--or anyone else reading this) live in an area where the power goes out regularly, it can really shorten the life of the batteries in your UPS. I live in the boonies and I'm on a rural electric co-op. The power in my area goes off many times during the year, especially during the winter, when snow downs the power lines, etc.. The outages can last from a minute or less to several hours and even a day. If the power happens to go out in the middle of the night while you're asleep (which it does frequently here), and you can't hear the buzzer in the UPS going off, it'll run until the battery simply dies out. Between my wife and I, we've replaced a whole lot of UPS batteries over the years which gets expensive after awhile. Turning off the PC every time you walk away is helpful, of course, but that gets old, too. ;>) And, no, I don't use the software that came with my UPS units.
But, one thing that I haven't seen mentioned in this thread (except peripherally by WaveScape) is that some UPS's provide regulated power. This is a worthwhile feature if you (or your dad) live in an area where power outages/fluctuations are common. Where there are outages, there are usually large power fluctuations, too. Giving your PC regulated power does extend it's life under these conditions. And, WaveScape is right: You really should have sine wave power regulation, not "simulated sine wave". This means a much more expensive UPS, of course.
If you're simply looking for good spike suppression, there's tons of devices available that will accomplish that. The higher the Joule rating, the better, as someone else has already mentioned. But, even with good spike suppression, a close lightening strike will probably take out any electronics that are plugged into a wall socket. I've lost two TV's and a variety of other electronic devices over the years to nearby lightening strikes (one hit the power pole between my neighbor's house and my house as I stood looking out the window. Quite, um, spectacular----and scary). As for me, nowadays I just go around the house and unplug any and all electronic equipment that is plugged into a wall socket at the first sign of T-storms. Even this isn't fool-proof, as others have said, but it's the safest thing I've found so far. ;>)
Best regards,
Bill