Which account to use was decided when SQL Server was installed, and the information was used to supply the installation procedure with the account name and/or password. Your DBA should know this information. In many corporate environments, there's a single domain account used for this purpose. If this is your own personal instance, you may want to Google search for how to find the SQL Server Agent Service account name. I don't know where that information resides, but I'd be very surprised if you had to go looking in the registry to find it.