Temporary user profile in Windows XP

I have an intetmittent problem on my Windows 2003 domain. Occasionally XP workstations (SP2) get an error when users log in. It states that the users profile could not be loaded and that a temporary profile is being used instead. Usually a reboot will resolve the issue and the user can then log in with their normal profile. Event log messages indicate that the ntuser.dat was in use and couldn’t be accessed when the user logged in. I’ve seen other forum posts about this on the web, but I haven’t been able to find anything conclusive as to the root cause. Any thoughts?