net stop mysql 先停下当前mysql服务 
c:\mysql-init.txt 内容如下
UPDATE mysql.user SET Password=PASSWORD('phpsir') WHERE User='root';
FLUSH PRIVILEGES;

然后 启动 mysql

C:\> "C:\Program Files\MySQL\MySQL Server 5.5\bin\mysqld.exe"  --defaults-file="C:\\Program Files\\MySQL\\MySQL Server 5.5\\my.ini" --init-file=C:\\mysql-init.txt  --console

参考 : http://dev.mysql.com/doc/refman/5.5/en/resetting-permissions.html