First, make sure you are logged in with Administer rights. Second, I suggest opening up Enterprise Manager, expanding your server, expand Management, expand Current Activity, and click on Process Info. This will show you what users are logged on and to what database they are using. You need to log any users off the database (KILL command). You might also have to set the database to SINGLE_USER (see BOL). Lastly, make sure your TSQL script is correct.
USE MASTER
DROP DATABASE <databasename>
You must USE MASTER when dropping any user database and you cannot drop the system databases (master, msdb, tempdb, model).
-Bill