I have a oracle installation on my windows machine but that was done a few months back and I could not recollect passwords for default sys and sysadmin records. The following trick helped reset passwords for these accounts
1. Go to Command prompt and type sqlplus /nolog. It should open SQL/> prompt.
2. On SQL/> prompt, type connect /as dba
Now you are logged in as dba, changing password is simple matter of
SQL> alter user sys identified by “newpassword”