UnloadUserProfile failed with LocalSystem Account (NT Service)

Hi There,
I have a program to loadUserprofile from LocalSystem Account(which running
service), and then UnloadUserProfile when not needed, It works fine until xp
sp2 come along. We still can load user profile but cannot Unload user
profile anymore. The error message is access denied. When I run the program
with administrator rights, then everything is fine too. It fails with
LocalSystem Account. Does anyone have explaination for it and work around.
Thnaks in advance!
Gordon