I'm going to second Mike's suggestion. Under $100 is a very tight budget, unless you have friends with old parts or stuff laying around the house. I built my dad an Atom 300 computer for him to use primarly for word processing and Quicken work, along with our Magic Jack, too, but that ended up running about $320 for everything.
You can get an ASUS Eee PC from newegg for $220 after rebate. It's the best price I've seen so far. Now mind you, I built a box for my mom's magic jack out of stuff I had lying around. I used an old ECS PM800-M2 motherboard, an Intel Celeron D, 512MB of DDR-333 RAM an old DVD-ROM and 160GB IDE HD. It works like a charm! All I had to do was order a case from newegg that was on sale for $20 with free shipping! Can't go wrong there eh?
You can find plenty of off lease P4 rigs for $130. The only thing with that is you're power efficiency is going to cost more in the end. A netbook or something with an Atom CPU might be a good bet. Ever think about a HTPC and just double the efforts?