CASE Alias In WHERE

Hello,

I found members of this group very helpful for my last queries.
Have one problem with CASE. I can use the column name alias in Order By Clause
but unable to use it in WHERE CLAUSE.
PLS TELL ME IF IT IS POSSIBLE TO USE IT IN WHERE CLAUSE AND SOME ALTERNATIVE.

QUERY:

SELECT
M.SECS =
CASE
WHEN NO_OF_SEC IS NULL THEN -1
WHEN NO_OF_SEC =0 THEN 1
ELSE NO_OF_SEC
END
FROM DOWNLOAD_MASTER M
WHERE M.SECS < 100
ORDER BY M.SECS



Hoping for a immediate reply.
thanks in advance