Starting at a user defined location in a string to find the nth occurance of a target string's starting location

EDIT: OK, I got Jay's set based method to work.  And I mistook Jay for Jay White (Page47) but I was wrong (but I should be given a pass, because it's just like what Mr. 47 would have written).  Now we have to ask, Jay who? 

Not anymore, thanks Jay

Anyway Thanks Jay.  Just as a note:  This requires a numbers table.  The concept of using this to eliminate recursion and cursors is sort of akin to turning a light on after stumbling around in the dark for so long (and even stumbling around in the light, since I couldn't employee this concept in my original shot at this).  The reasons to not use set based processing is becoming smaller and smaller on a daily basis.  Anyway, enjoy.

USE Northwind
GO

SET NOCOUNT ON
CREATE TABLE numbers(n int)
GO

DECLARE @x int
SET @x = 1

WHILE @x < 1001
  BEGIN
    INSERT INTO numbers(n) SELECT @x
    SELECT @x = @x + 1
  END
GO

CREATE FUNCTION udf_Instr
 (@str1 varchar(8000), @str2 varchar(8000), @start int, @Occurs int)
RETURNS int
AS
BEGIN
DECLARE @instr int
SELECT TOP 1  @instr = instr
  FROM (
  SELECT   d.*
  , (SELECT COUNT(1)
       FROM (SELECT DISTINCT CHARINDEX(@str1,SUBSTRING(@str2,@start,LEN(@str2)-@start+1),n) AS instr 
               FROM numbers
              WHERE n <= LEN(@str2))AS d2
      WHERE d2.instr <= d.instr) AS ct 

  FROM (SELECT DISTINCT CHARINDEX(@str1,SUBSTRING(@str2,@start,LEN(@str2)-@start+1),n) AS instr 
          FROM numbers
         WHERE n <= LEN(@str2)) AS d
 ) AS d
GROUP BY instr,ct
  HAVING ct <= @occurs
ORDER BY ct DESC
RETURN @instr
END

GO

SELECT dbo.udf_Instr('x','axbxcxdx',1,4)
GO

My Original shot:

CREATE FUNCTION udf_Instr
 (@str1 varchar(8000), @str2 varchar(8000), @start int, @Occurs int)
RETURNS int
AS
BEGIN
 DECLARE @Found int, @LastPosition int
 SET @Found = 0
 SET @LastPosition = @start - 1

 WHILE (@Found < @Occurs)
 BEGIN
  IF (CHARINDEX(@str1, @str2, @LastPosition + 1) = 0)
   BREAK
    ELSE
   BEGIN
    SET @LastPosition = CHARINDEX(@str1, @str2, @LastPosition + 1)
    SET @Found = @Found + 1
   END
 END

 RETURN @LastPosition
END
GO

SELECT dbo.udf_Instr('x','axbxcxdx',1,4)
GO

DROP FUNCTION udf_Instr
GO