diff --git a/lib/app.php b/lib/app.php
index 4e67da3c41..c2a850b6f6 100644
--- a/lib/app.php
+++ b/lib/app.php
@@ -59,7 +59,9 @@ class OC_APP{
 		// The rest comes here
 		$apps = OC_APPCONFIG::getApps();
 		foreach( $apps as $app ){
-			require( "apps/$app/appinfo/app.php" );
+			if( self::isEnabled( $app )){
+				require( "apps/$app/appinfo/app.php" );
+			}
 		}
 
 		self::$init = true;
@@ -68,6 +70,43 @@ class OC_APP{
 		return true;
 	}
 
+	/**
+	 * @brief checks whether or not an app is enabled
+	 * @param $app app
+	 * @returns true/false
+	 *
+	 * This function checks whether or not an app is enabled.
+	 */
+	public static function isEnabled( $app ){
+		if( 'yes' == OC_APPCONFIG::getValue( $app, 'enabled' )){
+			return true;
+		}
+
+		return false;
+	}
+
+	/**
+	 * @brief enables an app
+	 * @param $app app
+	 * @returns true/false
+	 *
+	 * This function set an app as enabled in appconfig.
+	 */
+	public static function enable( $app ){
+		OC_APPCONFIG::setValue( $app, 'enabled', 'yes' );
+	}
+
+	/**
+	 * @brief enables an app
+	 * @param $app app
+	 * @returns true/false
+	 *
+	 * This function set an app as enabled in appconfig.
+	 */
+	public static function disable( $app ){
+		OC_APPCONFIG::setValue( $app, 'enabled', 'no' );
+	}
+
 	/**
 	 * @brief makes owncloud aware of this app
 	 * @param $data array with all information
diff --git a/lib/installer.php b/lib/installer.php
index 7ab07bf507..02b71027cd 100644
--- a/lib/installer.php
+++ b/lib/installer.php
@@ -59,7 +59,7 @@ class OC_INSTALLER{
 		
 		if(!isset($data['source'])){
 			error_log("No source specified when installing app");
-			return;
+			return false;
 		}
 		
 		//download the file if necesary
@@ -67,13 +67,13 @@ class OC_INSTALLER{
 			$path=tempnam(sys_get_temp_dir(),'oc_installer_');
 			if(!isset($data['href'])){
 				error_log("No href specified when installing app from http");
-				return;
+				return false;
 			}
 			copy($data['href'],$path);
 		}else{
 			if(!isset($data['path'])){
 				error_log("No path specified when installing app from local file");
-				return;
+				return false;
 			}
 			$path=$data['path'];
 		}
@@ -92,7 +92,7 @@ class OC_INSTALLER{
 			if($data['source']=='http'){
 				unlink($path);
 			}
-			return;
+			return false;
 		}
 		
 		//load the info.xml file of the app
@@ -102,23 +102,33 @@ class OC_INSTALLER{
 			if($data['source']=='http'){
 				unlink($path);
 			}
-			return;
+			return false;
 		}
 		$info=OC_APP::getAppInfo($extractDir.'/appinfo/info.xml');
 		$basedir=$SERVERROOT.'/apps/'.$info['id'];
 		
 		//check if an app with the same id is already installed
-		if(is_dir($basedir)){
+		if(self::isInstalled( $info['id'] ))){
 			error_log("App already installed");
 			OC_HELPER::rmdirr($extractDir);
 			if($data['source']=='http'){
 				unlink($path);
 			}
-			return;
+			return false;
 		}
+
+		//check if the destination directory already exists
++		if(is_dir($basedir)){
++			error_log("App's directory already exists");
++			OC_HELPER::rmdirr($extractDir);
++			if($data['source']=='http'){
++				unlink($path);
++			}
++			return false;
++		}
 		
 		if(isset($data['pretent']) and $data['pretent']==true){
-			return;
+			return false;
 		}
 		
 		//copy the app to the correct place
@@ -128,7 +138,7 @@ class OC_INSTALLER{
 			if($data['source']=='http'){
 				unlink($path);
 			}
-			return;
+			return false;
 		}
 		OC_HELPER::copyr($extractDir,$basedir);
 		
@@ -150,6 +160,23 @@ class OC_INSTALLER{
 		
 		//set the installed version
 		OC_APPCONFIG::setValue($info['id'],'installed_version',$info['version']);
+		OC_APPCONFIG::setValue($info['id'],'enabled','no');
+		return true;
+	}
+
+	/**
+	 * @brief checks whether or not an app is installed
+	 * @param $app app
+	 * @returns true/false
+	 *
+	 * Checks whether or not an app is installed, i.e. registered in apps table.
+	 */
+	public static function isInstalled( $app ){
+
+		if( null == OC_APPCONFIG::getValue( $app, "installed_version" )){
+			return false;
+		}
+
 		return true;
 	}
 
diff --git a/lib/setup.php b/lib/setup.php
index e9cb3da686..43ead7b96a 100644
--- a/lib/setup.php
+++ b/lib/setup.php
@@ -147,6 +147,7 @@ class OC_SETUP {
 
 					$info=OC_APP::getAppInfo("$SERVERROOT/apps/$app/appinfo/info.xml");
 					OC_APPCONFIG::setValue($app,'installed_version',$info['version']);
+					OC_APPCONFIG::setValue($app,'enabled','yes');
 				}
 
 				//create htaccess files for apache hosts