XML Editor in Visual Studio 2008

Hi all,

When I try to open xml files (files with the extensions like .xml, .config
etc.)
VS 2008 does not open them with the xml editor like it used to.
And when i right click to choose from the available options (open with), it
is not listed. I'm not sure when it started to happen. Maybe after I
installed SP1, but I might be wrong. In addition, when I click
Tools->Options, in the Options dialog box, when I click Text Editor->XML in
the tree view on the left, if I click either Formatting or Miscellaneous,
instead of showing the property page on the right, it says "an error occured
loading this property page".
Any ideas on what happened here and how I can fix it?

Thanks,
Ben
.