Opening an xlsx file results in Excel creating a hidden file with the same name preceded by "~$". So if one of these Excel files are open at the time you retrieve the content of the directory, you will also get the temp file.
Add a filter that excludes hidden files and your issue is fixed.
Example:
FileAttributes attributes = File.GetAttributes(path);
if((attributes & FileAttributes.Hidden) == FileAttributes.Hidden)
{
// Hidden file, just skip it
}
From http://msdn.microsoft.com/en-us/library/system.io.file.getattributes.aspx