Deleted Flag/Unique Constraint

Is it possible to declare a unique constraint in combination with a deleted flag?

For example, if I have a table like this:

CREATE TABLE
(
  ID NOT NULL PRIMARY KEY,
  Key VARCHAR(32) NOT NULL,
  Value VARCHAR(32) NOT NULL,
  Deleted INT NOT NULL DEFAULT 0
);

can I declare a unique constraint that only checks Key and Value when Deleted = 0?

Thanks,
Bryan