I've had my bad experiences with big box bicycles. Yes, I think they're mostly junk, but I don't dismiss them outright. Were it not for cheap department store bikes my barely making it family would never have been able to get me a bike at all when I was growing up out in the middle of the desert. I probably wouldn't have become the enthusiast I am now without having that "gateway drug". I'm going to disagree with the majority of the very good experts on here and say that department story bikes have their place, that being said I don't intend to buy another one.
If you only have $100 to spend you really are better off buying a used but good bike off of Craig's list, or ask around, there may be someone at your company, church, social circle of any type willing to part with a good bike for that amount or even less if it's gathering dust. In the world of bicycles old does not equal bad. A bike that was a good bike 30 years ago but is well maintained is a good bike today, you may need new tires and some fresh grease.
If a Wal-Mart bike is your only real option do not buy the fanciest bike they have. When it comes to department store bikes "better is worse". Let me explain my reasoning. Bicycles are made from many different components, to keep cost low when Wal-Mart orders a huge order of bikes manufactures have to cut corners at every opportunity to keep production cost low. If you have a fancy multi-speed bike that means you have crappy derailers, crappy brakes, crappy everything to deal with and your chances of failed components and even injury from failed components (read my first paragraph about growing up nearly poor on these types of bikes - it happened to me) increases.
If you must get a Wal-Mart bike get a simple cruiser. Their cheap La Jolla Cruiser, while still a cheap crappy bike has very few components that can fail. It's a single speed coaster brake bike, net result you're less likely to get injured by it and it will last a little longer.
My mother bought a multi-speed Schwinn bike from Wal-Mart. I honestly have to say it's not that bad, it's only got one derailer, a rear one, and it's Shimano. It may not be Shimano's best offering, but it's not bad overall, for the amount of riding my mother does it's good enough. I've given it a spin once or twice and it's really smooth/easy riding. If she put the type of miles on a bike I do I would very quickly ask her to consider getting an Electra, a Specialized, or even a Schwinn for an actual Schwinn bike shop, but the $170 ish she spent on it doesn't seem like a waste from my perspective, she's an occasional recreation weekend rider - the category this type of bike covers.
Look for used first. $100 is limiting even when choosing from Wal-Mart.