It's a common American dream, to build your own business and watch it grow and thrive. It has happened to many people, as their little ventures that started in garages and sheds became companies like Hewlett-Packard (NYSE:HPQ), Disney (NYSE:DIS), and Apple (NASDAQ:AAPL). Even The Motley Fool was born in a shed, although we can't claim the same lofty heights as those other companies (yet!).

But hold on. If you're thinking of joining their ranks, don't get the idea that it's easy. According to the U.S. Small Business Administration, about half of small businesses fail within the first five years. Even big businesses can fail -- think of Circuit City, for example, or Lehman Brothers. Retailers like Talbots (NYSE:TLB) and Office Depot (NYSE:ODP) are struggling today, with falling revenues and substantial amounts of debt.

That doesn't mean you shouldn't try to start a small business -- but do so with your eyes open, after having done a lot of research into why so many businesses fail. (Some key reasons: poor locations, financing difficulties, inexperienced owners.) And don't take risks you don't need to.

Keep retirement money for retirement
For instance, many people these days are using 401(k) money to start a business, and it's very risky. Granted, it may be your best source of funds, but borrowing during tough times has its own dangers.

If your business doesn't work out, you can end up losing all those savings altogether. Even if it does work out, you may have been able to earn more simply by leaving that money where it was. 

Here's what I mean. Let's say that in your 401(k), you grew a $10,000 nest egg into about $26,000 from age 40 to 50, at an average of 10% per year. Then, at age 50, you took that $26,000 and invested it in your business. (We'll assume no penalties paid in this example, as some people are able to avoid penalties.) After five years, the business is off the ground and you get back $26,000 in cash. Then you reinvest it, and it resumes its 10% growth until age 65.

Under those assumptions, you'll end up with about $67,000. If you'd left that money alone, though, giving it five more years, you'd have ended up with $108,000, fully $41,000 more! Meanwhile, you're taking a very real risk of ending up with absolutely nothing.

Your retirement money is vital to your future. Learn to make the most of your 401(k). If you'd like to set yourself up for a non-painful retirement, try our Rule Your Retirement newsletter service for free, with full access to all past issues and investing recommendations.

Longtime Fool contributor Selena Maranjian owns shares of Apple. Walt Disney is a Motley Fool Inside Value selection. Walt Disney and Apple are Motley Fool Stock Advisor recommendations. Try our investing newsletters free for 30 days. The Motley Fool is Fools writing for Fools.