public static SPUser Find(SPWeb inWeb, string loginName)
        {
            try
            {
                if (string.IsNullOrEmpty(loginName))
                    return null;

                string searchedLogin = loginName;
                SPWeb web = inWeb.IsRootWeb ? inWeb : inWeb.ParentWeb;
                SPUser result = null;
                SPSecurity.RunWithElevatedPrivileges(delegate
                                                         {
                                                             using (var elevatedSite = new SPSite(web.Site.ID))
                                                             {
                                                                 using (var elevatedWeb = elevatedSite.OpenWeb(web.ID))
                                                                 {
                                                                     result = elevatedWeb.EnsureUserUnsafe(searchedLogin);
                                                                 }
                                                             }
                                                         });
                return result;
            }
            catch
            {
                return null;
            }

------------------------------

        public static SPUser EnsureUserUnsafe(this SPWeb web, string logonName)
        {
            UnsafeUpdate uu = null;
            try
            {
                uu = new UnsafeUpdate(web);
                return web.EnsureUser(logonName);
            }
            catch (SPException ex)
            {
                throw new SPException("error ensure user " + logonName, ex);
            }
            finally
            {
                if (uu != null)
                {
                    uu.Dispose();
                }
            }
        }