After I installed Microsoft Office 2010, it seems like I have lost my file associations for XML Copy Editor. If I check Start > Default Programs, I can see that XML Copy Editor is associated with .xml and .xsl files, but such files do not any longer have the XML Copy Editor icon for example. When I double click them, they once opened up in the editor but I got the error message "There was a problem sending the command to the program". When I double click them now, the files open in Internet Explorer although they are associated with XML Copy Editor.
It did not help to reinstall XML Copy Editor. So maybe this is a Windows issue, or Office issue?