Try this:
SELECT Team, Points, GoalDiff,
(SELECT Count(*)+1 FROM tblTeams t1 where t1.points > t.points or (t1.points >= t.points and t1.goaldiff > t.goaldiff)) as Position
FROM tblTeams t
ORDER BY t.Points DESC , t.GoalDiff DESC;
It may not look pretty, but it seems to work pretty good.