The software XP Keep Per User Display Settings steps in and provides a way so that every user can work with his own display settings. This is achieved by running the application in the background. It automatically keeps a log of all user settings and activities applying and saving display settings whenever a user logs in, logs off or switches accounts.

The information are stored in the registry where they are loaded and saved when the need arises.

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XP Keep Per User Display Settings
XP User Display Settings (similar software but without support for Fast User Switching)

The tool xp user display settings stores the following information for every user on a single machine separately and loads those settings once the user logs into his account. Every user may choose the resolution, refresh rate and color depth that he likes to work with ignoring the settings of other users who have a account on that machine as well.

Xp user display settings does not seem to support fast user switching, that is at least what the author is stating at the products homepage. He suggest that you user XP Keep per user display settings instead which seems to support fast user switching. It would be best if you would take a look at the read me file which comes included with both programs to know what they do and how to remove them completely if something goes wrong.