Before installing Chrome check that the installation directory doesn't exist (or can be deleted).

chrome/installer/util/util_constants.h

 // Copyright (c) 2006-2008 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 //
 // Defines all install related constants that need to be used by Chrome as
 // well as Chrome Installer.
 
 #ifndef CHROME_INSTALLER_UTIL_UTIL_CONSTANTS_H__
 #define CHROME_INSTALLER_UTIL_UTIL_CONSTANTS_H__
 
@@ skipping 18 matching lines @@
   CHROME_RUNNING,        // Chrome currently running (when trying to uninstall)
   UNINSTALL_CONFIRMED,   // User has confirmed Chrome uninstall
   UNINSTALL_SUCCESSFUL,  // Chrome successfully uninstalled
   UNINSTALL_FAILED,      // Chrome uninstallation failed
   UNINSTALL_CANCELLED,   // User cancelled Chrome uninstallation
   UNKNOWN_STATUS,        // Unknown status (this should never happen)
   RENAME_SUCCESSFUL,     // Rename of new_chrome.exe to chrome.exe worked
   RENAME_FAILED,         // Rename of new_chrome.exe failed
   EULA_REJECTED,         // EULA dialog was not accepted by user.
   EULA_ACCEPTED,         // EULA dialog was accepted by user.
-  EULA_ACCEPTED_OPT_IN   // EULA accepted wtih the crash optin selected.
+  EULA_ACCEPTED_OPT_IN,  // EULA accepted wtih the crash optin selected.
+  INSTALL_DIR_IN_USE     // Installation directory is in use by another process
 };
 
 // These are distibution related install options specified through command
 // line switches (see below) or master preference file (see
 // chrome/installer/util/master_preference.h). The options can be combined,
 // so they are bit flags.
 enum InstallOption {
   // A master profile file is provided to installer.
   MASTER_PROFILE_PRESENT  = 0x1,
   // The master profile file provided is valid.
@@ skipping 38 matching lines @@
 extern const wchar_t kChromeNewExe[];
 extern const wchar_t kChromeDll[];
 extern const wchar_t kSetupExe[];
 extern const wchar_t kInstallerDir[];
 
 extern const wchar_t kUninstallStringField[];
 extern const wchar_t kUninstallDisplayNameField[];
 }  // namespace installer_util
 
 #endif  // CHROME_INSTALLER_UTIL_UTIL_CONSTANTS_H__