What I'm looking to do is permission a Windows PC (ideally XP but if a later OS version is required so be it) so that a select group of users can unlock the machine, logging the current user out. This something that a Local Admin for the machine would be able to do from a locked screen, however, I'd like to avoid granting Local Admin rights to this group of users if I can.

The background here is that this machine is 80% used by one person but is treated as a 'shared machine' on days that the primary user is not around. It's usefull that everybody using the machine can carry on using their own accounts with all the personalisations they are used to. I'd also like to void logging the primary user out every night. No one else in the firm has to put up with that and she does use the machine herself most of the time.