How to use alias column name in where clause in SQL Server

You can’t use aliased columns in a WHERE clause. You can try using a derived table. Perhaps something like this (sorry, not tested):

SELECT * 
FROM (
    SELECT SQRT( POWER( cast(Program_Latitude as float) 
                        - cast('41.5126237' as float), 2) 
               + POWER( cast(Program_Longitude as float) 
                        - cast('-81.6516411' as float), 2)
               ) * 62.1371192    AS DistanceFromAddress 
    from tblProgram 
    ) mytable
WHERE DistanceFromAddress < 2