对于oracle10g来说：
1.如果不记得sys用户的密码了，采用如下方法可以修改密码：
(1)打开cmd，输入sqlplus /nolog，回车。
(2)输入“conn / as sysdba”;
(3)输入“alter user sys identified by 新密码;”（新密码必须以字母开头，另外每条SQL语句后得分号不能忘）
其他用户方式同理
2.(1)在cmd中启动sqlplus；
  (2)用sqlplus/as sysdba登录，使其连接到:
 Oracle Database  10g Express Edition Release 10.2.0.1.0 - Production
  (3)执行ALTER USER DBSNMP ACCOUNT UNLOCK；
  (4) 从新用SQL/PLUS登录，设置用户名、密码；
 
附：Oracle 10g 默认安装带来的用户名/密码
 
Username
Password
Description
See Also
CTXSYS
CTXSYS
The Oracle Text account
Oracle Text Reference
DBSNMP
DBSNMP
The account used by the Management Agent component of Oracle Enterprise Manager to monitor and manage the database
Oracle Enterprise Manager Grid Control Installation and Basic Configuration
LBACSYS
LBACSYS
The Oracle Label Security administrator account
Oracle Label Security Administrator's Guide
MDDATA
MDDATA
The schema used by Oracle Spatial for storing Geocoder and router data
Oracle Spatial User's Guide and Reference
MDSYS
MDSYS
The Oracle Spatial and Oracle interMedia Locator administrator account
Oracle Spatial User's Guide and Reference
DMSYS
DMSYS
The Oracle Data Mining account.
Oracle Data Mining Administrator's Guide
Oracle Data Mining Concepts
OLAPSYS
MANAGER
The account used to create OLAP metadata structures. It owns the OLAP Catalog (CWMLite).
Oracle OLAP Application Developer's Guide
ORDPLUGINS
ORDPLUGINS
The Oracle interMedia user. Plug-ins supplied by Oracle and third party format plug-ins are installed in this schema.
Oracle interMedia User's Guide
ORDSYS
ORDSYS
The Oracle interMedia administrator account
Oracle interMedia User's Guide
OUTLN
OUTLN
The account that supports plan stability. Plan stability enables you to maintain the same execution plans for the same SQL statements. OUTLN acts as a role to centrally manage metadata associated with stored outlines.
Oracle Database Performance Tuning Guide
SI_INFORMTN_SCHEMA
SI_INFORMTN_SCHEMA
The account that stores the information views for the SQL/MM Still Image Standard
Oracle interMedia User's Guide
SYS
CHANGE_ON_INSTALL
The account used to perform database administration tasks
Oracle Database Administrator's Guide
SYSMAN
CHANGE_ON_INSTALL
The account used to perform Oracle Enterprise Manager database administration tasks. Note that SYS and SYSTEM can also perform these tasks.
Oracle Enterprise Manager Grid Control Installation and Basic Configuration
SYSTEM
MANAGER
Another account used to perform database administration tasks.