SELECT    curr.AssetYear, curr.AssetMonth,
    SUM(curr.Total + ISNULL(prev.Total, 0)) AS Asset_Total
FROM    (
       SELECT  Year(Date) AS AssetYear, 
           Month(Date) AS AssetMonth, 
           Sum(Value) AS Total
          FROM    Asset
       WHERE   date between @beginDate And @endDate
       GROUP BY Year(Date), Month(Date)
    ) 
    AS curr LEFT JOIN
    (
       SELECT  Year(Date) AS AssetYear, 
           Month(Date) AS AssetMonth, 
           Sum(Value) AS Total
          FROM    Asset
       WHERE   date between @beginDate And @endDate
       GROUP BY Year(Date), Month(Date)
    ) 
    AS prev ON curr.AssetYear = prev.AssetYear 
        AND curr.AssetMonth > prev.AssetMonth
GROUP BY curr.AssetYear, curr.AssetMonth