(MONEY Magazine) – What would happen to your finances if you had twins, or your spouse stopped working, or you gave up a steady salary to start a business? One of the illusions that you get from personal-finance software--especially if you're one of those dutiful souls who conscientiously track every dollar that you spend or save--is that you are in total control. But I have seen the best of savers turned into deficit financers in the wake of a major life event because they failed to anticipate how much it would change their spending patterns.

Everyone who has used them knows that the top financial management programs can analyze what has happened from every conceivable angle. But how good are the programs at helping you plan for what might happen? To find out, I decided to test the "what if..." potential of four leading personal-finance software programs--Quicken, Managing Your Money, Microsoft Money and Kiplinger's Simply Money. Here's what I found.

--Getting a grip on future expenses. All of the programs I examined have default categories for tallying current expenses--items like housing, clothing, auto, insurance, telephone, education and so forth. They also let you create your own categories. And that's the feature that can be useful in planning for future expenses that come with a life change.

For example, suppose you want to see what would happen if you added a new member to your family. By creating a category, baby, you can produce a hypothetical budget that factors in all your likely child-related costs. Under baby, you can then make subcategories for child care, clothing, diapers, other baby supplies and all the rest. Before Junior arrives, plug estimates of those costs into the software and compare your pre-baby and post-baby budgets side by side. What you learn once you've entered all your numbers may surprise you.

Some expectant parents I know who recently performed this exercise found that after the baby arrived they would no longer be saving an admirable 12% of their annual incomes. Instead, they discovered, they'd be saving nothing and spending an unfortunate 5% more than they earned--unless they made some substantial changes in their budget.

Before I make this process sound too easy, a few words of warning: For the digitally dumbfounded, figuring out how to create new budget categories can be frustrating. Simply Money, for example, is anything but simple when it comes to modifying categories: You must exit the main transaction account where you enter your spending data and return to the home screen in order to make any alterations.

Nor should you expect much advice on preparing for life's surprises. Of the four programs, only Quicken (and just the deluxe version that retails for $59.95) offers much useful help. Its "Life Events" section asks a series of questions to help forecast the costs and other financial ramifications of having a child, getting married, retiring and so on.

For the tax portion of your "what if..." scenario, summon your 1995 TurboTax or Kiplinger TaxCut from the musty corner of your hard drive where you banished it on April 16. Both programs have planning features that allow you to estimate your future income tax.

--Knowing whether your income will keep pace with expenses. Just as smart companies try to project their income and expenses at least a year ahead, forward-thinking people can do much the same thing. Those expectant parents I mentioned were surprised to learn from running the numbers that if the lower-income spouse worked half time rather than full time, they would come out ahead, despite the smaller paycheck. (The money they'd save on child care, commuting and income taxes would more than make up for it.)

If you are leaving your job to start a small business, to take a different job or to retire, these programs can also help you see how your income will be affected. Three of the four programs allow you to project your income and expenses month by month a full year into the future. (Simply Money will do the calculations but won't print them out.) Microsoft Money can project a year ahead but doesn't let you enter figures that vary from month to month.

Inevitably, of course, unanticipated expenses will push your budget out of whack some months. But don't let that distract you. What's important is that you hit your earning, spending and savings targets over longer time periods, such as six or 12 months. If you can manage that, you will be in pretty good shape. But don't get smug about it. Despite the best-laid plans of computer mice and men, life still surprises. Remember the expectant parents I mentioned earlier? After they had run all the numbers on the new baby, they got some news from the doctor: They were going to have triplets.

Author of Personal Finance for Dummies (IDG Books, $16.99), Eric Tyson is a financial counselor who teaches personal finance at the University of California-Berkeley.