Should Be Easy - Help Formatting A Conditional Query.

SQL Server 2000. Here's what I have so far. The section of the query I need help with is highlighted in blue.
 
CREATE PROCEDURE dbo.GetByVersion
(
 @targetVersion varchar(30),
 @product varchar(50)
)
AS
 SET NOCOUNT ON;
SELECT *  FROM  MyTable
WHERE (product = @product)
AND
  CASE 
    WHEN @targetVersion='' THEN (targetVersion='')
    ELSE (targetVersion LIKE @targetVersion + '%')
  END
GO
 
I get a syntax error in the Stored Procedure editor on an equal sign in this line:
    WHEN @targetVersion='' THEN (targetVersion='')

What I want is this (in psuedocode):
  if @targerVersion is blank
    search for records where the targetVersion column = blank
  else
    search for records where the targetVersion column starts with @targetVersion
 
Can anyone offer any suggestions as to how I might modify my query to do what I want? Any help is very much welcome - Thanks in advance! :-)