Can't change my display properties

Hi. I recently installed a halloween desktop theme (from a microsoft site)
on my computer. It worked just fine, but today when I tried to go back to
the Windows XP theme, it will not change back all the way. The windows and
buttons are the classic style, while everything else is the XP theme. When I
try to change it, my computer freezes up and I end up having to restart. I
tried to uninstall the halloween theme, but had to get rid of it piece by
piece, it wasn't listed in the add/remove programs area. Did something from
the theme I downloaded mess up my ability to change the theme back completely
to XP? Or, is there something simple I'm missing that someone else knows.
I'd appriciate any help. Thanks.