Select Middle Rows in SQL Server

Try something like this....

SELECT TOP (50) PERCENT *
FROM (
        SELECT TOP (20) PERCENT 
                      [col1]
                     ,[col2]
        FROM [table]
        ORDER BY [col1] DESC
     )T
ORDER BY [col1] ASC

You can use a simple good old not in:

SELECT TOP 10 PERCENT [col1], [col2] 
FROM [table] 
WHERE [col1] NOT IN (
    SELECT TOP 10 PERCENT [col1]
    FROM [table]
    ORDER BY [col1] DESC
)
ORDER BY [col1] DESC