One method of actually using a Windows Function in a WHERE clause

CTE_WindowFunction.sql

WITH MahQuery AS (
    SELECT  DummyId,
            PartitionId,
            ROW_NUMBER() OVER (PARTITION BY PartitionId ORDER BY DummyId DESC) AS LastN
    FROM    dbo.DummyTable
)
SELECT  DummyId,
        PartitionId,
        LastN
FROM    MahQuery
WHERE   LastN <= 2;