Can I query what username is being used in my SQL db's?

hi sql guru's.

so I am in need of changing an admin password but I am afraid that doing so might mess up sql in some way shape or form thus breaking user applications along the way.

I want to ensure that the admin account was not used in any db or for any other purpose such are reporting.  Is there a command that will query everything and inform me what usernames are being used?  this would be for db's running on SQL 2005

thanks!