I have an excellent Excel spreadsheet (also have it for a MAC, in Numbers) to do your family budget.
It has all of the formulas figured out for you, you just input your income and then your expenses and you can see how much you are left with (or negative) at the end of the month (and at the end of each week and pay period). You can add in categories for your personal needs, too.
I use it all of the time, and it is a lifesaver!
Edited to add: Here's the link to the website that it comes from originally (although I had it emailed to me by a friend) and you can download it directly from here:
home (momsfinance)