Select Query is very slow

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SQL-Server

Hi All

[SQL Server 2008]
I have a table(DATALOG_4) with 10 Millions records. It contains 18 columns. The column DATE_TIME is a primary key column. We have created Non-Clustered Index on DATE_Time because we are using this column in WHERE Clause.

The issue is, It takes more than 20 seconds for even after I created Non-Clustered index.

We are using the following Query:

SELECT DATE_TIME, [794] AS CT, [61] AS BID,
         ROW_NUMBER() OVER (PARTITION BY [61] ORDER BY  DATE_TIME DESC) AS Row_No
   FROM DATALOG_4 (NOLOCK)
   WHERE
        DATE_TIME >= '2012-11-01 12:00:00' and DATE_TIME <= '2012-12-31 11:59:59'

Please anyone suggest me, How to make the select Query fast?