Index: wp-admin/includes/file.php
===================================================================
--- wp-admin/includes/file.php	(revision 20475)
+++ wp-admin/includes/file.php	(working copy)
@@ -80,9 +80,9 @@
 	$home = get_option( 'home' );
 	$siteurl = get_option( 'siteurl' );
 	if ( $home != '' && $home != $siteurl ) {
-		$wp_path_rel_to_home = str_replace($home, '', $siteurl); /* $siteurl - $home */
-		$pos = strrpos($_SERVER["SCRIPT_FILENAME"], $wp_path_rel_to_home);
-		$home_path = substr($_SERVER["SCRIPT_FILENAME"], 0, $pos);
+		$wp_path_rel_to_home = str_ireplace( $home, '', $siteurl ); /* $siteurl - $home */
+		$pos = strripos( str_replace( '\\', '/', $_SERVER['SCRIPT_FILENAME'] ), $wp_path_rel_to_home );
+		$home_path = substr( $_SERVER['SCRIPT_FILENAME'], 0, $pos );
 		$home_path = trailingslashit( $home_path );
 	} else {
 		$home_path = ABSPATH;