The credentials used will depend on the protocol you wish to use for your Windows servers. There is a section in the SIM user guide that explains all the OS requirements to get things discovered properly.
For Windows you have two options: SNMP and WBEM. SNMP will require the Management Agents at minimum plus the SNMP service configured to allow the SIM server to communicate (community name, SIM server listed as authorized host, etc). WBEM requires the HP WBEM Provider and agents at a mininum, then admin credentials (a special user, local admin, or a domain user with admin priveleges).
I assume since adding admin creds got it working, you've got things set up using WBEM.