All databases must have a transaction log, even on SQL Server 2000. Are you trying to remove a log file you are no longer using, but there will still be another log file available? If so, you may have transactions in the log you're trying to detach. If your database is in Simple recovery, try running the command CHECKPOINT and see if you can then detach the log. If your database is in FULL recovery you'll need to run a log backup, then run CHECKPOINT and then, probably, run another log backup. But, that all assumes you're trying to remove one of many log files, not the only log file on the database.