The obvious thing to do is to find a good used bike. Distinguishing a good used bike from a bad used bike can be tricky, particularly if you're not really familiar with the details of bikes produced over the last 20 years or so. If you can find a bike with a good brand name (Trek, Giant, Specialized, Jamis, Kona) and leave enough in your budget to take it to an LBS for a solid tune-up, you might be able to do this. Bike fit is really important in this case, and that means more than just sitting on the bike and making sure you can reach the pedals.
Your other option is a new bike. The $400 price cap limits your options a lot, especially once you leave room in the budget for things like lock and helmet. Resist the temptation to go to a department store or sporting goods store. Go to your local bike shop and see what they have. You may find a good bargain on a 2008 bike as the 2009's come out. Ride a few bikes, and find one that feels good. Make sure they pay attention to fit.