Index: branches/RC/core/install.php
===================================================================
diff -u -N -r11071 -r11074
--- branches/RC/core/install.php	(.../install.php)	(revision 11071)
+++ branches/RC/core/install.php	(.../install.php)	(revision 11074)
@@ -448,13 +448,23 @@
 						$this->errorMessage = 'Please select module(-s) to ' . ($this->currentStep == 'choose_modules' ? 'install' : 'upgrade');
 					}
 
-					$found['In-Portal'] = in_array('kernel', $modules);
-					$found['Proj-Base'] = in_array('proj-base', $modules);
+					// check interface modules
+					$available_modules = $selected_modules = Array ();
+					$interface_modules = Array ('In-Portal', 'Proj-Base');
+					foreach ($interface_modules as $interface_module) {
+						$module_folder = strtolower($interface_module == 'In-Portal' ? 'kernel' : $interface_module);
 
-					if (!$found['In-Portal'] && !$found['Proj-Base']) {
-						$this->errorMessage = 'Please ' . ($modules ? 'also ' : '') . 'select "' . implode('" or "', array_keys($found)) . '" as interface module';
+						if (file_exists(MODULES_PATH . '/' . $module_folder)) {
+							array_push($available_modules, $interface_module);
+						}
+
+						$module_selected[$interface_module] = in_array($module_folder, $modules);
 					}
 
+					if (!$module_selected['In-Portal'] && !$module_selected['Proj-Base']) {
+						$this->errorMessage = 'Please ' . ($modules ? 'also ' : '') . 'select "' . implode('" or "', $available_modules) . '" as interface module';
+					}
+
 					$status = $this->errorMessage == '';
 					break;
 			}
Index: branches/RC/core/install/step_templates/choose_modules.tpl
===================================================================
diff -u -N -r10924 -r11074
--- branches/RC/core/install/step_templates/choose_modules.tpl	(.../choose_modules.tpl)	(revision 10924)
+++ branches/RC/core/install/step_templates/choose_modules.tpl	(.../choose_modules.tpl)	(revision 11074)
@@ -19,6 +19,7 @@
 <?php
 	$module_tpl = ob_get_clean();
 
+	$first_time = $this->GetVar('step') != $this->currentStep; // data from this step was not submitted yet
 	$selected = $this->GetVar('modules');
 	if (!$selected) {
 		// preselect interface modules
@@ -49,7 +50,7 @@
 		}
 		else {
 			// preserve user selected checked status
-			$checked = in_array($module, $selected) ? 'checked="checked"' : '';
+			$checked = in_array($module, $selected) || $first_time ? 'checked="checked"' : '';
 		}
 
 		$error_msg = $prerequisites_errors ? implode('<br />', $prerequisites_errors) : '';