My favorite at-home workout equipment is a recumbent (sp?) bike. It works more of your body than a regular stationary bike, plus it's easier on your knees, ankles, and feet. My mom has arthritis in her knees and she still uses her bike every day. Plus most of them are quiet enough you can do it while you watch TV or while the baby sleeps. You can get a very basic model for under $100 just about anywhere, Walmart, Sears, I've even seen them at Walgreens! You could also get a fancy one with a digital readout that tracks your pulse and tells you how many calories you've burned, but those tends to run several hundred dollars more.
Good luck with whatever you go with, don't give up!