As long as you don't reformat your drive everything should be fine.
It is only asking you to supply your own personal login name and password to create an account for you (Mac OS X is a multi-user systems). There are no serial numbers needed, but don't forget your password!
Once installed you can use the system preferences to select Mac OS 9.1 as your start up OS, and then in 9.1 use the Startup Disk control panel to get back to Mac OS X.