If you set up a maintenance plan to cleanup and delete old files from a folder and it doens't delete the files (a common problem as it turns out), create a new maintenance plan and add an 'Execute T-SQL Statement' task.

The following will delete BAK and TRN files, enter it in the T-SQL statement window:

declare @dt datetime
select @dt=getdate()-[number of day's files to keep, i.e. 14 to delete files over 14 days old]
EXECUTE master.dbo.xp_delete_file 0,N'[file folder]',N'BAK',@dt
EXECUTE master.dbo.xp_delete_file 0,N'[file folder]',N'TRN',@dt

Set up a reccuring schedule to run once a day and save the new plan.

Old files will now be deleted and your hard drive won't fill up.