Doug Knox's XP_QuickRes.exe System Tray Utility to Change Screen Resolution page contains:

Version 2.0.5 - This small Visual Basic 6 utility will put an icon in your System Tray and allow you to select one of seven preset resolution settings and six preset refresh rates (60, 70, 72, 75, 85 and 100, as long as your monitor supports them). Changing of the color depth is not supported in this release. For you gamers, 640x480 is on the list, as well! Some resolution/refresh rates may not be available, due to your current color depth settings. Simply right click on the icon in the System Tray to access the features.

Download xp_quickres.zip.

NOTE: See XP_UserDisplay.exe freeware allows all users of a Windows XP computer to have their own screen resolution.