Can't Delete Registry Key

I am trying to install Service Pack 2 on my dad's computer and am having a problem. Every time I tried to install it I got an "Access is Denied" error. I searched for everything I could find on that error and found that it is usually the result of incorrect permissions on the registry. I found the registry key in question: HKEY_CLASSES_ROOT\.dvr-ms which appears to reference a type of file that is used when Windows XP Media Center Edition records a show. The only thing is, this is Windows XP Home. When I try to delete the key I get an error: "An error occurred while deleting this key." I also can't set permissions so I know that's not the problem. I'm assuming the key is corrupted somehow, and it's keeping me from installing SP2. Is there any way to fix/remove this messed up key or to simply override the SP2 install to ignore it?

Thanks!
-corndog