I share a home computer with my wife and 3 year old daughter, Rosemary. As a computer nerd, I can't go below 1280x1024 on the screen resolution, while my wife likes 1024x768 and Rosemary uses 640x480.
Windows XP doesn't support per user screen resolutions: http://support.microsoft.com/?kbid=832295
Turns out there are some freeware products that do this. I checked out a few and settled on XPKeepPerUserDisplaySettings. It didn't seem to "take" the first time I made the settings for each user, but now it works like a charm.
 Rosemary only uses 640x480 because most children's computer programs are so poorly written they only work in 640x480. She amazes me - no problems at finding the little red X on a form at 1024x768. When her favorite site, www.uptoten.com, added a tiny link so parents would have to log their kids on and just break down and buy some non-free stuff, it took her a 30 second lesson to learn how to find the little "no thanks, take me to the free site" link. I can't wait until she's 4 and can take over BIOS upgrades for the family.