Index: wp-admin/network.php
===================================================================
--- wp-admin/network.php	(revision 23258)
+++ wp-admin/network.php	(working copy)
@@ -312,7 +312,8 @@
 	$hostname          = get_clean_basedomain();
 	$slashed_home      = trailingslashit( get_option( 'home' ) );
 	$base              = parse_url( $slashed_home, PHP_URL_PATH );
-	$wp_dir_from_root  = preg_replace( '#^' . preg_quote( $_SERVER['DOCUMENT_ROOT'], '#' ) . '#', '', ABSPATH );
+	$document_root     = str_replace( '\\', '/', realpath( $_SERVER['DOCUMENT_ROOT'] ) );
+	$wp_dir_from_root  = preg_replace( '#^' . preg_quote( $document_root, '#' ) . '#', '', str_replace( '\\', '/', ABSPATH ) );
 	$wp_siteurl_subdir = trailingslashit( '/' . preg_replace( '#^' . preg_quote( $base, '#' ) . '#', '', $wp_dir_from_root ) );
 	$rewrite_base      = ! empty( $wp_siteurl_subdir ) ? ltrim( trailingslashit( $wp_siteurl_subdir ), '/' ) : '';