Try these query to shrink the log file (copied from http://mywinsysadm.wordpress.com/2011/09/22/sharepoint-sharepoint_config_log-is-large/):
Using SQL Server Management Studio:
1) Make a backup of the SharePoint_Config database
2) Execute the following query:

SQL Server 2008:

USE [master]
GO
ALTER DATABASE[SharePoint_Config] SET RECOVERY SIMPLE WITH NO_WAIT
GO
USE [SharePoint_Config]
GO
DBCC SHRINKFILE (‘SharePoint_Config_Log’)
GO
ALTER DATABASE[SharePoint_Config] SET RECOVERY FULL WITH NO_WAIT
GO
USE [SharePoint_Config]
GO