说明：win7系统，已经装过一个安装版的mysql（服务没有启动），然后又安装了一个免安装版的mysql，然后启动该免安装版的mysql后，用root用户无法登陆（因为想着root用户默认密码是空，但是却无法登陆），如下所示：
D:>cd D:MySQL Server 5.1in
D:MySQL Server 5.1in>mysql -u root mysql
ERROR 1045 (28000): Access denied for user 'root'@'localhost' (using password: NO)
解决办法：
D:MySQL Server 5.1in>mysql -p
Enter password:
Welcome to the MySQL monitor. Commands end with ; or g.
Your MySQL connection id is 9
Server version: 5.5.24 MySQL Community Server (GPL)
Copyright (c) 2000, 2010, Oracle and/or its affiliates. All rights reserved.
This software comes with ABSOLUTELY NO WARRANTY. This is free software,
and you are welcome to modify and redistribute it under the GPL v2 license
Type 'help;' or 'h' for help. Type 'c' to clear the current input statement.
将密码重新设置（设为空）
mysql> set password for 'root'@localhost=password('');
Query OK, 0 rows affected (0.00 sec)
mysql> exit
Bye
再次以root用户（密码为空）则可以登陆：
D:MySQL Server 5.1in>mysql -u root
Welcome to the MySQL monitor. Commands end with ; or g.
Your MySQL connection id is 10
Server version: 5.5.24 MySQL Community Server (GPL)
Copyright (c) 2000, 2010, Oracle and/or its affiliates. All rights reserved.
This software comes with ABSOLUTELY NO WARRANTY. This is free software,
and you are welcome to modify and redistribute it under the GPL v2 license
Type 'help;' or 'h' for help. Type 'c' to clear the current input statement.
我想出现这个原因，估计是因为我之前装的这个mysql，root用户密码修改了，不是非空了。