Create parameter value that returns several values

Hi,

I've done this before but it's been a while and I just can't remember how I did it.

I've got a query that returns data filtered on the course entered e.g.

SELECT [srs_mcr].mcr_code  
FROM [srs_mcr]
WHERE [srs_mcr].mcr_code  = @Course 

However, I want the @Course parameter to only have 2 values; one that inputs all mcr_codes (all courses) and one that inputs all mcr_codes excluding the ones that are in a table called 'tbl_ExcludedCourses' as these courses need to be excluded from the league table for university tariffs (only want undergrad courses - not foundation courses) 

So I want the @Course parameter to just have 2 values - 'All' and say 'ValidCourses'.

What do i need to do to incorporate all the mcr_codes into these values?

Cheers, AndySugs