I think the problem has to do with my attempt to insatll oracle.
Since then my limited user accounts were disabled and my recycle bin was corrupted.
I managed to fix this by running system restore.
However, my limited user accounts and the guest account can only use the windows classic theme (boring!)
When I am logged in as one of the limited account users I have tried to chose the luna theme from the themes tab. This results in an error message that says that the theme cannot be found. But when I navigate to the location C:\WINDOWS\Resources\Themes, everything seems to be there.
The strange thing is that on my account (the adminstrator) I still have the Win XP luna theme.
I have tried searching the MS support pages but have not found a solution.