You could try logging in as root. At the login window use root as the name and use the same password you use for the admin account. Reset the admin password using any one of the usual methods.
If you have a client OS Installer DVS (10.6, or 10.6.3 for example) or any other OS installer DVD that can boot your server you can use the Reset Password Utility. In other words you don't need the Server Installer Disk if all you want to do is reset the admin password.
You really should try and find the Server Installation Disk. A reinstall would be impossible without it.