Index: wp-admin/includes/schema.php
===================================================================
--- wp-admin/includes/schema.php	(revision 6568)
+++ wp-admin/includes/schema.php	(working copy)
@@ -149,7 +149,7 @@
 	global $wpdb, $wp_db_version;
 
 	$schema = ( isset($_SERVER['HTTPS']) && strtolower($_SERVER['HTTPS']) == 'on' ) ? 'https://' : 'http://';
-	$guessurl = preg_replace('|/wp-admin/.*|i', '', $schema . $_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI']);
+	$guessurl = eregi($schema . $_SERVER['HTTP_HOST'], $_SERVER['REQUEST_URI']) ? preg_replace('|/wp-admin/.*|i', '', $_SERVER['REQUEST_URI']) : preg_replace('|/wp-admin/.*|i', '', $schema . $_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI']);
 	add_option('siteurl', $guessurl);
 	add_option('blogname', __('My Blog'));
 	add_option('blogdescription', __('Just another WordPress weblog'));
Index: wp-admin/includes/upgrade.php
===================================================================
--- wp-admin/includes/upgrade.php	(revision 6568)
+++ wp-admin/includes/upgrade.php	(working copy)
@@ -23,7 +23,7 @@
 	if ( defined('WP_SITEURL') && '' != WP_SITEURL )
 		$guessurl = WP_SITEURL;
 	else
-		$guessurl = preg_replace('|/wp-admin/.*|i', '', $schema . $_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI']);
+		$guessurl = eregi($schema . $_SERVER['HTTP_HOST'], $_SERVER['REQUEST_URI']) ? preg_replace('|/wp-admin/.*|i', '', $_SERVER['REQUEST_URI']) : preg_replace('|/wp-admin/.*|i', '', $schema . $_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI']);
 
 	update_option('siteurl', $guessurl);