squirrely when your PC asks for more power than it can easily handle. Plus some of them don't have proper Voltage Regulation, so the power's not much cleaner than what comes out the wall socket. And without a fast enough switchover time after power failure, that can be pretty hard on your system too. Most should at least sufficiently isolate you from lightning strikes, though.

Myke advocates APC. I would too, but I don't have that kind of budget, or a server room where I can appropriate any that get upgraded. I've got a CyberPower 1500VA for my main system plus a tiny AMD box in the same room, and a CyberPower 585VA for my dad's system. I'm fairly happy with both.

Just make sure it has "Automatic Voltage Regulation" and sufficient network / phone jacks. I lost some equipment in a lightning strike, because it carried over the CAT5 cable. If you've got a cable modem with naught but a USB port, I guess you'd look for Coax protection too. Me, I just protect the CAT5 link coming out of it, becase the cable company replaces a failed modem anyway.