Keep in mind when deleting files from code the typical override allowing users to delete anything in their path, whether it is marked readonly or not, will prevent your IO process from continuing and throw a nice little generic “access denied” error.

To resolve this, simply remove the readonly attribute on the file prior to deletion.

     if (File.GetAttributes(fullfilename) == FileAttributes.ReadOnly)
                    {
                        File.SetAttributes(fullfilename, FileAttributes.Normal);
                    }

                    File.Delete(fullfilename);

Alternatively, using FileSystemInfo (unmodified courtesy of Ken White):

private static void DeleteFileSystemInfo(FileSystemInfo fsi)
{
    fsi.Attributes = FileAttributes.Normal;
    var di = fsi as DirectoryInfo;

    if (di != null)
    {
        foreach (var dirInfo in di.GetFileSystemInfos())
            DeleteFileSystemInfo(dirInfo);
    }

    fsi.Delete();
}

References
Wordpress (Imran Akram), http://imak47.wordpress.com/2009/01/15/how-to-remove-readonly-attribute-from-a-file/
StackOverflow (Ken White), http://stackoverflow.com/questions/667381/programatically-run-cmd-exe-as-adminstrator-in-vista-c