Just in case, IF you already have an index on ( YearValue, MonthValue ), then this should be extremely fast:
SELECT
MAX(YearValue) AS YearValue, MAX(MonthValue) AS MonthValue --or some computation to turn this into a datetime
FROM dbo.myTable
WHERE
YearValue = ( SELECT MAX(YearValue) FROM dbo.myTable )
SQL DBA,SQL Server MVP(07, 08, 09) A socialist is someone who will give you the shirt off *someone else's* back.