How slow? A 333MHz is the minimum for
Hauppauge's products. Anything with USB2.0 ought to be more than fast enough. Just watching TV takes very little CPU time really, but you certainly wouldn't want to be doing anything else while watching on a slow machine, and a slow machine can't handle the PVR functions with a non-hardware encoding device.
Anything too slow to just watch TV isn't likely to even have USB1.1 ports.
Just check Newegg's listings for others, probably cheaper ones available, and the system requirements should be easy to find.