When you use the Microsoft Remote Installation Services (RIS), by default the Administrator password is set to null (blank) during the installation. You can, however, let the user set a password during the final GUI portion of installation. Perform the following steps:
AdminPassword = *
AdminPassword = ""
During installation, the system will prompt the user to type an Administrator password. You should test this change to ensure that it works correctly.
As a side note, instead of "" you could type a password (e.g., AdminPassword = "fred"), which sets the Administrator password to the password you specify and doesn't prompt the user. However, this password travels as clear text, so I don't recommend this approach.
The Windows 2000 Resource Kit describes another option (although not well!). You can use a Custom Installation Wizard (CIW) and let the user type in a password. However, this approach is quite complex.