And I use the PIIIs in my signature everyday for my work. Never any limitations. If I ever do video editing (eventually?) I know I would need something faster, but for regular "office work," they're quite fast and very stable...
I used a PIII 850 Win98SE machine to run my business for a couple years. Video capture and editing was a real problem(Win98SE was the first OS with native firewire support), FAT 32 file size limitation, etc. Analog video capture with an ATI AIW card was a major headache.
I looked through the local Craigslist free stuff just now, one 15" monitor with no power cable and broken printer was the only computer thing I saw. I'd give it a try, it's free. Your system might be the call for somebody who doesn't have a computer and can't look at Craigslist, maybe a local free paper?