aspnet local user account rights to c:\windows\temp automatically removed

I have a client that uses a dot net web app on a windows 2003 server.
Every day they get a dotnet message from the users browser that tells
us that the aspnet user does not have security rigths to c:\windows
I look at the security tab and see that the local user aspnet is gone,
re-add it and things work. the next day the same thing happens again.

They are using active directory and the admin tells me that he didn't
change anything?