T-SQL CASE statement in the WHERE clause of a query

I am working on a report that pulls data from a stored procedure.  The application prompts the user for parameters and then passes them into the stored procedure so the result set can be tailored to the user's preferences.

In the parameters dialog, the user can select to filter by a minimum and maximum number of days or months.  (If you're wondering, It's a banking industry thing.  90 days and 3 months are two entirely different things).  Anyway, months vs. days two mutually exclusive checkboxes (as opposed to a radio button) where the user can also choose not to filter by date range at all.

In my WHERE clause, I want to be able to do something like this:
SELECT <some stuff>
FROM <a bunch of tables>
WHERE <filtering done by other parameters> AND
        (IF @P1 = 1 OR @P2 = 1 THEN
                CASE (@P1 = 1)
                        THEN <call some function>
                CASE (@P2 = 1)
                        THEN <call some other function>)

The idea here is to do different calculations depending on whether we're filtering by days or months.  The CASE statement shouldn't even evaluate at all if neither option is taken.   When Googling this on the web, I get mixed results.  Some sites say you can't do that, and other say you can.  If you can do this, what would be the appropriate syntax?  How would I avoid (or deal with) the situation where the CASE statement would make the WHERE clause evaluate to something like WHERE <yada> AND  (IF 1 THEN 1), which is total nonsense.

Oh, and BTW, I am using SQL Server 2000.Start Free Trial