If you're using SQL Server 2008 then you can combine the columns like this:
SELECT CAST(CAST(your_date_column AS DATE) AS DATETIME) +
CAST(your_time_column AS TIME)
FROM your_table
For earlier versions of SQL Server then this is what you'll need:
SELECT DATEADD(day, 0, DATEDIFF(day, 0, your_date_column)) +
DATEADD(day, 0 - DATEDIFF(day, 0, your_time_column), your_time_column)
FROM your_table
Try this one also:
Convert(datetime, Convert(char(10), MYDATETIMEFIELD, 103) + ' ' + Convert(char(8), MYTIMEFIELD, 108), 103)