The purpose of this application is to help employees to ensure that they do not have too much or too little income tax withheld from their pay. It is not a replacement for Form W-4, but most people will find it more accurate and easier to use than the worksheets that accompany Form W-4. You may use the results of this program to help you complete a new Form W-4, which you will submit to your employer.

http://www.irs.gov/individuals/article/0,,id=96196,00.html