adding rows and columns....

i heard about this problem,but i am not sure if its correct.anybody who knows this problem may please correct the problem....however it may be right also (Itwasntme) so here it goes:

consider an 8x8 chessboard.now every squares of the board is filled with either **-1** or **+1**.the product of each rows and columns are then computed.

how many arrangements like this can be made such that the sum of all the products of the columns equals that of the sum of the products of the rows ?

please help!