Maybe theres a kind of problem in your COMPONENT mult8X8... :-/

Attach this file for a closer look.
(pls with extension *.vhd, and you will see some syntax highlighting 
magic)


BTW: do you know, that VHDL has a multiplcation operator, the '*'?

There are no previously attached files...  :-/
At least I didn't find any.

However:
This is your "problem"

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     when 17 =>    

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         if (A(7) xor B(7)) = '1' then      

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           product <= not ACC(15 downto 0) + '1';

You assign the final value after 17 clocks. So until then the result is 
-U-ndefined.

A solution for you is to assign a default value to Product like this:

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entity mult8X8 is

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        port (Clk, St: in std_logic;

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            Mplier,Mcand : in std_logic_vector(7 downto 0);

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            Product: out std_logic_vector(15 downto 0) := (others => '0'); -- some little magic here

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            Done: out std_logic);

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end mult8X8;

BTW: do you know, that VHDL has a multiplication operator, the '*'?
So you could smply write: Product <= Mplier * Mcand;