I Just installed ubuntu and did most of the default options, including installing GRUB. I had windows before, and I still want it to be the default OS. I'm afraid my family will be confused when the GRUB screen comes up, and I want GRUB to automatically boot Windows. How could I do this?

See the 'default' option, e,g, 'default 0' will auto-boot the first entry.

Thanks a lot. Also, how would I get GRUB to reappear when I want to boot into linux?

If you set the default to Windows AND hide the grub boot screen, then you will never see the options. You have at least 2 choices:
unhide the grub screen, set the timeout to maybe 2-3 seconds. You family can just see it go by and ignore it, but YOU will know to hit the down arrow the instant you see the screen.
Let Windows boot on its own and make a floppy to boot Linux.

I have toyed with how one might write a script to tell grub what to do on a re-boot. Easy to do from Linux, but not so easy when re-booting from Windows.
(The idea would be simply to use the script to modify grub.config and then reboot.)

Lastly, I was just reading about the Acronis Boot Manager--maybe they have some other tricks.....

Since pressing the enter key when the grub menu is displayed will override the timeout, why not just set a reasonable timeout to give yourself time to take a few breaths? Instruct your family that windows is the default system. When you see this screen (the grub menu), just press enter to begin booting windows.

Even for die-hard windows users that can't be too terribly irritating.

Or, better yet. Set up user accounts in Linux for them, and encourage them to explore Linux.

In our household, I am the SA, and the user base is me, my wife, and the pet-sitter. The rules are simple: You don't like the way the central computer is configured, then get your own. I will configure it any way you want--including Windows.
But--I will not be the one that drives to the store and actually buys a copy of Windows.