Hi

Try the following steps

1) you have to delete the password file that corresponds to the Instance

2) Recreate the password file using the ORAPWD command from the command prompt do not use oradim that command is used to create a Serive for the Instance on Windows NT and Windows 98 the syntax is as follows

orapwd file= password= entries=5

Entries is optional that specified how many users can have sysdba or sysoper privilege but remember if you want to add more then 5 after creating the password file you will have do delete the password file and recreate the password file.

Good Luck