The best account to login to the Oracle Enterprise Manager Database Control (EMDC) is the SYSMAN user account; with it, you are logging in as the proper EMDC super user. Use NORMAL, not AS SYSDBA.
Alternatively, you can use SYSTEM (specifying AS SYSDBA).
This is per 11.1 manual: http://docs.oracle.com/cd/B28359_01/install.111/b32002/rev_precon_db.htm#autoId1
The same manual describes how to, being in EMDC, convert another "normal" database user (the one you use in sqlplus) into an EMDC-capable user. Or, from within EMDC, you can create a new user: a database user that is both EMDC-capable and sqlplus-capable from the beginning.
As SYSMAN wouldn't have a password by default after install, you need to assign it. Just become an OS user which is in
dba group (typically this OS user is called
oracle). This is for Linux, but it is similar enough in Windows. Then issue:
connect / as sysdba
-- here this should let you in without password
-- if this displays "idle instance", you probably need to STARTUP
alter user sysman identified by "MyPass!@#$";
alter user sysman account unlock;