While I'm assuming this is a homework problem, and I normally don't help with those, this was interesting enough that I couldn't stop myself. 🙂

You have a number of flow-of-logic errors. For example, if you look at where you have your increment on @a, you'll notice that you'll never reach that if @i is not evenly divisible by @a. So, the moment you try to divide 3 by 2, it never increments @a to 3, but just loops. Even if you fix that, check what'll happen to your increment for @i when it hits 4. Since 4 is the first non-prime number, it has a problem with where the increment is in relation to the check for @count.

Try this version:

DECLARE

@i INT,

@a INT,

@count INT

SET @i = 1

WHILE (@i <= 500)

BEGIN

SET @count = 0

SET @a = 1

WHILE (@a <= @i)

BEGIN

IF (@i % @a = 0)

SET @count = @count + 1

SET @a = @a + 1

END

IF (@count = 2)

PRINT @i

SET @i = @i + 1

END