I'm not sure that XP supports changing the resolution when different users log in (I don't know why, it was the same way in NT 4.0 and 2000 before that). What I suggest (and this takes a bit of work to set up, but works OK) is that you download and install
Multires. Then create a shortcut to the executable with the various commandline options needed to set a certain resolution, color depth, and refresh rate, and place this shortcut file in the Startup folder in the Start Menu for the user that resolution corresponds to.
For example, if you had one user who ran his display at 1280x1024, 32 bit color, 85 Hz refresh rate, the commandline
"C:\Program Files\MultiRes\Multires.exe" /1280,1024,32,85
entered in the Target box of a shortcut will set the display to those settings. If the other user ran his (or her) display at 1024x786, 16 bit color, 100 Hz refresh rate, the commandline in the Target box would look like
"C:\Program Files\MultiRes\Multires.exe" /1024,768,16,100
These shortcut files would then be placed in their respective users' Startup folders, and when they log on the resolution should change.
Be warned that I have never tried this with fast user switching, so I don't know what would happen in that situation.
Also, the application path should be changed to whatever the proper location for the Multires program is on your harddrive.