To detach, you will surely need to take the db offline. The easiest way is right click on the db and click take offline(make sure there are no important users on it). In case you have users on the db which you think can be killed, then run the kill users routine available on sqlservercentral, or anywhere else on the internet(for SQL 2005) after you have clicked the take offline option. Once the db is offline, you can do the detach and the subsequent attach after relocating useing the management studio itself...

Thanks..