There is many different way to restore a database.
One of them been using the regular restore command in Query script, the other is by using Enterprise Manager, when the database already exist.
The other way you can restore a database when you already have Dat files under SQL 7 and SQL 2000 would be to use the SP_attach_db. What that do is connect the database to your SQL server.
NOTE: you may have to recreate the users in your database in order to assigh the proper security.
If you are using SQL 6.5 what you can do after you have installed the a new version you can create a new database with the same name as the one you add before. Stop your services and replace the dat files with the one you would like to use.
Note: This process would not work under more resent version of SQL server.